There currently exist two different function names for getting the entry count
of maps, where offmaps offset and string maps use `num_entries` and OID maps use
`size`. In most programming languages with built-in map types, this is simply
called `size`, which is also shorter to type. Thus, this commit renames the
other two functions `num_entries` to match the common way and adjusts all
callers.

The current way of looking up an entry from a map is tightly coupled with the
map implementation, as one first has to look up the index of the key and then
retrieve the associated value by using the index. As a caller, you usually do
not care about any indices at all, though, so this is more complicated than
really necessary. Furthermore, it invites for errors to happen if the correct
error checking sequence is not being followed.

Introduce a new high-level function `git_strmap_get` that takes a map and a key
and returns a pointer to the associated value if such a key exists. Otherwise,
a `NULL` pointer is returned. Adjust all callers that can trivially be
converted.

Currently, one would use the function `git_strmap_insert` to insert key/value
pairs into a map. This function has historically been a macro, which is why its
syntax is kind of weird: instead of returning an error code directly, it instead
has to be passed a pointer to where the return value shall be stored. This does
not match libgit2's common idiom of directly returning error codes.

Introduce a new function `git_strmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert all callers of
`git_strmap_insert` to make use of it.

The current way of looking up an entry from a map is tightly coupled with the
map implementation, as one first has to look up the index of the key and then
retrieve the associated value by using the index. As a caller, you usually do
not care about any indices at all, though, so this is more complicated than
really necessary. Furthermore, it invites for errors to happen if the correct
error checking sequence is not being followed.

Introduce a new high-level function `git_oidmap_get` that takes a map and a key
and returns a pointer to the associated value if such a key exists. Otherwise,
a `NULL` pointer is returned. Adjust all callers that can trivially be
converted.

Currently, one would use either `git_oidmap_insert` to insert key/value pairs
into a map or `git_oidmap_put` to insert a key only. These function have
historically been macros, which is why their syntax is kind of weird: instead of
returning an error code directly, they instead have to be passed a pointer to
where the return value shall be stored. This does not match libgit2's common
idiom of directly returning error codes.Furthermore, `git_oidmap_put` is tightly
coupled with implementation details of the map as it exposes the index of
inserted entries.

Introduce a new function `git_oidmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert all trivial callers of
`git_oidmap_insert` and `git_oidmap_put` to make use of it.

The current way of looking up an entry from a map is tightly coupled with the
map implementation, as one first has to look up the index of the key and then
retrieve the associated value by using the index. As a caller, you usually do
not care about any indices at all, though, so this is more complicated than
really necessary. Furthermore, it invites for errors to happen if the correct
error checking sequence is not being followed.

Introduce a new high-level function `git_offmap_get` that takes a map and a key
and returns a pointer to the associated value if such a key exists. Otherwise,
a `NULL` pointer is returned. Adjust all callers that can trivially be
converted.

Currently, there is only one caller that adds entries into an offset map, and
this caller first uses `git_offmap_put` to add a key and then set the value at
the returned index by using `git_offmap_set_value_at`. This is just too tighlty
coupled with implementation details of the map as it exposes the index of
inserted entries, which we really do not care about at all.

Introduce a new function `git_offmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert the caller to make use
of it instead.

The current way of looking up an entry from a map is tightly coupled with the
map implementation, as one first has to look up the index of the key and then
retrieve the associated value by using the index. As a caller, you usually do
not care about any indices at all, though, so this is more complicated than
really necessary. Furthermore, it invites for errors to happen if the correct
error checking sequence is not being followed.

Introduce new high-level functions `git_idxmap_get` and `git_idxmap_icase_get`
that take a map and a key and return a pointer to the associated value if such a
key exists. Otherwise, a `NULL` pointer is returned. Adjust all callers that can
trivially be converted.

Currently, one would use the function `git_idxmap_insert` to insert key/value
pairs into a map. This function has historically been a macro, which is why its
syntax is kind of weird: instead of returning an error code directly, it instead
has to be passed a pointer to where the return value shall be stored. This does
not match libgit2's common idiom of directly returning error codes.

Introduce a new function `git_idxmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert all callers of
`git_idxmap_insert` to make use of it.

The currently existing function `git_idxmap_resize` and
`git_idxmap_icase_resize` do not return any error codes at all due to their
previous implementation making use of a macro. Due to that, it is impossible to
see whether the resize operation might have failed due to an out-of-memory
situation.

Fix this by providing a proper error code. Adjust callers to make use of it.

Some callers were still using the tightly-coupled pattern of `lookup_index` and
`valid_index` to verify that an entry exists in a map. Instead, use the more
high-level `exists` functions to decouple map users from its implementation.

Currently, our headers need to leak some implementation details of maps due to
their direct use of indices in the implementation of their foreach macros. This
makes it impossible to completely hide the map structures away, and also makes
it impossible to include the khash implementation header in the C files of the
respective map only.

This is now being fixed by providing a high-level iteration interface
`map_iterate`, which takes as inputs the map that shall be iterated over, an
iterator as well as the locations where keys and values shall be put into. For
simplicity's sake, the iterator is a simple `size_t` that shall initialized to
`0` on the first call. All existing foreach macros are then adjusted to make use
of this new function.

To compute whether there are objects missing in a packfile, the indexer keeps
around a map of OIDs that it still expects to see. This map does not store any
values at all, but in fact the keys are owned by the map itself. Right now, we
free these keys by iterating over the map and freeing the key itself, which is
kind of awkward as keys are expected to be constant.

We can make this a bit prettier by inserting the OID as value, too. As we
already store the `NULL` pointer either way, this does not increase memory
usage, but makes the code a tad more clear. Furthermore, we convert the
previously existing map iteration via indices to make use of an iterator,
instead.

To relieve us from memory pressure, we may regularly call `cache_evict_entries`
to remove some entries from it. Unfortunately, our cache does not support a
least-recently-used mode or something similar, which is why we evict entries
completeley at random right now. Thing is, this is only possible due to the map
interfaces exposing the entry indices, and we intend to completely remove those
to decouple map users from map implementations. As soon as that is done, we are
unable to do this random eviction anymore.

Convert this to make use of an iterator for now. Obviously, there is no random
eviction possible like that anymore, but we'll always start by evicting from the
beginning of the map. Due to hashing, one may hope that the selected buckets
will be evicted at least in some way unpredictably. But more likely than not,
this will not be the case. But let's see what happens and if any users complain
about degraded performance. If so, we might come up with a different scheme than
random removal, e.g. by using an LRU cache.

Remove the low-level interface that was exposing implementation details of
`git_strmap` to callers. From now on, only the high-level functions shall be
used to retrieve or modify values of a map. Adjust remaining existing callers.

Remove the low-level interface that was exposing implementation details of
`git_offmap` to callers. From now on, only the high-level functions shall be
used to retrieve or modify values of a map. Adjust remaining existing callers.

Remove the low-level interface that was exposing implementation details of
`git_oidmap` to callers. From now on, only the high-level functions shall be
used to retrieve or modify values of a map. Adjust remaining existing callers.

Remove the low-level interface that was exposing implementation details of
`git_idxmap` to callers. From now on, only the high-level functions shall be
used to retrieve or modify values of a map. Adjust remaining existing callers.