[super! 2/6] Give each convention a single overload type - #67

A convention used to carry a tuple-like overload_types, so adding a
convention had to search Cs for an existing convention with the same
is_direct and dispatch_type and merge the overload lists into it. That
merge is the only reason add_conv_t, merge_conv_t, add_conv_reduction,
and conv_specialization_t exist.

Replace overload_types with a single overload_type, so add_convention
contributes one convention per overload and merging a convention into Cs
becomes deduplicating concatenation. Duplicates are still collapsed, so
build() is unaffected at both compile time and run time.

Accessors still have to be formed per dispatch type, or a dispatch with
several overloads would contribute several accessor bases instead of one
overload set. Regroup the conventions by dispatch type via conv_group
and conv_groups_merge_t just before generating the accessors, which also
keeps overload shadowing working.

Generalize the reduction helpers to carry extra arguments (reduction_t)
and add flattening_merge_t, which merge_tuples_t and conv_groups_merge_t
are both expressed in terms of.

This changes the ProBasicConvention and ProConvention requirements, so a
hand-written convention type must be updated.