integer: use type-generic overflow intrinsics for size_t - #7406

Build the vendored libgit2 for `target_os = "wasi"` (tested on
wasm32-wasip1; wasip2 is untested). There is no system libgit2 to link
against, so WASI always builds the vendored copy. In git2, path
conversions accept WASI's `OsStr`, and `Repository::open_ext` and
`discover_path` join ceiling directories themselves, since std's
`env::join_paths` is unsupported on WASI.

WASI preview 1 lacks several things libgit2's unix build assumes:

- mmap: build with `NO_MMAP`, so pack windows are read into memory.
- threads: leave `GIT_THREADS` undefined, and reject the -threads
  targets, where git2's `Send` types could then race inside libgit2.
- network connections: replace `streams/socket.c` with a stream that
  always fails, and reject the `https` and `ssh` features.
- user and process identity: wasi/compat.c defines getuid and friends
  (uid 0, pid 1). wasi-libc has no pwd.h, so wasi/include/pwd.h
  provides one; libgit2 only consults it when setuid, so the home
  directory comes from $HOME.

On wasm32, `size_t` is `unsigned long` but `SIZE_MAX == UINT_MAX`, so
libgit2's `integer.h` passes a `size_t *` to `__builtin_uadd_overflow`.
The types have the same width, so clang's error is downgraded for WASI
as a stopgap until libgit2 uses the type-generic builtins
(libgit2/libgit2#7406).

Add a CI job that builds for wasm32-wasip1 with wasi-sdk.

Build the vendored libgit2 for `target_os = "wasi"` (tested on
wasm32-wasip1; wasip2 is untested). There is no system libgit2 to link
against, so WASI always builds the vendored copy. In git2, path
conversions accept WASI's `OsStr`, and `Repository::open_ext` and
`discover_path` join ceiling directories themselves, since std's
`env::join_paths` is unsupported on WASI.

WASI preview 1 lacks several things libgit2's unix build assumes:

- mmap: build with `NO_MMAP`, so pack windows are read into memory.
- threads: leave `GIT_THREADS` undefined, and reject the -threads
  targets, where git2's `Send` types could then race inside libgit2.
- network connections: replace `streams/socket.c` with a stream that
  always fails, and reject the `https` and `ssh` features.
- user and process identity: wasi/compat.c defines getuid and friends
  (uid 0, pid 1). wasi-libc has no pwd.h, so wasi/include/pwd.h
  provides one; libgit2 only consults it when setuid, so the home
  directory comes from $HOME.

On wasm32, `size_t` is `unsigned long` but `SIZE_MAX == UINT_MAX`, so
libgit2's `integer.h` passes a `size_t *` to `__builtin_uadd_overflow`.
The types have the same width, so clang's error is downgraded for WASI
as a stopgap until libgit2 uses the type-generic builtins
(libgit2/libgit2#7406).

libz-sys before 1.1.30 builds zlib for WASI without default allocators
(rust-lang/libz-sys#273), so every object read fails. Require 1.1.30.

Add a CI job that builds for wasm32-wasip1 with wasi-sdk and runs the
tests under wasmtime. MSRV only builds: before Rust 1.94, std on WASI
can't canonicalize paths and its remove_dir_all fails on what the
tests create. For the tests:

- std has no temporary directory on WASI, so tests create theirs with
  `test::tempdir()`, which points tempfile at /tmp there, as tempfile's
  own tests do. The runner preopens /tmp.
- Test helpers with only unix and windows variants (`realpath`,
  `ostr_to_cstr`, file URLs, the symlink in `repo_status`) now cover
  WASI, and `search_path` joins paths itself.
- Skipped on WASI: `compile_fail` (trybuild runs cargo),
  `non_utf8_branch` (WASI paths are UTF-8), and two tests that remove
  directories through paths ending in a slash, which wasmtime rejects
  (bytecodealliance/wasmtime#11524).