Auto merge of #118024 - notriddle:notriddle/search-speed, r=GuillaumeGomez

rustdoc-search: optimize unifyFunctionTypes

Final profile output:
https://notriddle.com/rustdoc-html-demo-5/profile-4/index.html

This PR contains three commits that improve performance of this hot inner loop: reduces the number of allocations, a fast path for the 1-element basic query case, and reconstructing the multi-element query case to use recursion instead of an explicit `backtracking` array. It also adds new test cases that I found while working on this.

r? `@GuillaumeGomez`

Author: bors