Auto merge of #112233 - notriddle:notriddle/search-unify, r=GuillaumeGomez

rustdoc-search: clean up type unification and "unboxing"

This PR redesigns parameter matching, return matching, and generics matching to use a single function that compares two lists of types.

It also makes the algorithms more consistent, so the "unboxing" behavior where `Vec<i32>` is considered a match for `i32` works inside generics, and not just at the top level.

Author: bors