This significantly shrinks the pre-compressed search index:
$ du -h searchindex-old.js searchindex-new.js
26M searchindex-old.js
19M searchindex-new.js
And shrinks the search index even after it's gzipped:
$ du -h searchindex-old.js.gz searchindex-new.js.gz
4.5M searchindex-old.js.gz
3.3M searchindex-new.js.gz
This change requires a newer version of mdBook, with
https://github.com/rust-lang/mdBook/pull/1637