match lowering: Rename `MatchPair` to `MatchPairTree` In #120904, `MatchPair` became able to store other match pairs as children, forming a tree. That has made the old name confusing, so this patch renames the type to `MatchPairTree`. This PR also includes a patch renaming the `test` method to `pick_test_for_match_pair`, since it would conflict with the main change. r? `@Nadrieril`