Rollup merge of #144320 - lolbinarycat:rustdoc-search_index-BTreeMap-str, r=GuillaumeGomez

rustdoc: avoid allocating a temp String for aliases in search index

Here's the optimization I talked about in rust-lang/rust#143988 (comment)

I got around the Serialize issue using the newtype pattern.  The wrapper type could be factored out into a helper that would work with anything that impls `AsRef<&str>`, but I'm not sure if that would be helpful anywhere else.

r? ``````@GuillaumeGomez``````

Author: fmease