Sema: rewrite peer type resolution

The existing logic for peer type resolution was quite convoluted and
buggy. This rewrite makes it much more resilient, readable, and
extensible. The algorithm works by first iterating over the types to
select a "strategy", then applying that strategy, possibly applying peer
resolution recursively.

Several new tests have been added to cover cases which the old logic did
not correctly handle.

Resolves: ziglang#15138
Resolves: ziglang#15644
Resolves: ziglang#15693
Resolves: ziglang#15709
Resolves: ziglang#15752