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.
This feature extends rustdoc to support the syntax that most users will
naturally attempt to use to search for diverging functions.
Part of #60485
It's already possible to do this search with `primitive:never`, but
that's not what the Rust language itself uses, so nobody will try it if
they aren't told or helped along.
This makes sense, since the search index has the information in it,
and it's more useful for function signature searches since a
function signature search's item type is, by definition, some type
of function (there's more than one, but not very many).
Based on https://github.com/rust-lang/rust/pull/108200, for the same
rationale.
> This replaces the existing Levenshtein algorithm with the
> Damerau-Levenshtein algorithm. This means that "ab" to "ba" is one change
> (a transposition) instead of two (a deletion and insertion). More
> specifically, this is a restricted implementation, in that "ca" to "abc"
> cannot be performed as "ca" → "ac" → "abc", as there is an insertion in the
> middle of a transposition. I believe that errors like that are sufficiently
> rare that it's not worth taking into account.
Before this change, searching `prinltn!` listed `print!` first, followed
by `println!`. With this change, `println!` matches more closely.
bors
