Rollup merge of rust-lang#149149 - yotamofek:wip/rustdoc/search_index/misc-cleanups, r=GuillaumeGomez

[rustdoc] misc search index cleanups

Bunch of misc cleanups to search index generation (on the rustdoc side). Perf isn't improved unfortunately, but I do think most of these changes are improvements to readability/style.

Probably easier to review commit by commit.

Author: Zalathar