rustdoc: use a trie for name-based search

Preview and profiler results
----------------------------

Here's some quick profiling in Firefox done on the rust compiler docs:

- Before: https://share.firefox.dev/3UPm3M8
- After: https://share.firefox.dev/40LXvYb

Here's the results for the node.js profiler:

- https://notriddle.com/rustdoc-html-demo-15/trie-perf/index.html

Here's a copy that you can use to try it out. Compare it with [the nightly].
Try typing `typecheckercontext` one character at a time, slowly.

- https://notriddle.com/rustdoc-html-demo-15/compiler-doc-trie/index.html

[the nightly]: https://doc.rust-lang.org/nightly/nightly-rustc/

The fuzzy match algo is based on [Fast String Correction with
Levenshtein-Automata] and the corresponding implementation code in [moman]
and [Lucene]; the bit-packing representation comes from Lucene, but the
actual matcher is more based on `fsc.py`. As suggested in the paper, a
trie is used to represent the FSA dictionary.

The same trie is used for prefix matching. Substring matching is done with a
side table of three-character[^1] windows that point into the trie.

[Fast String Correction with Levenshtein-Automata]: https://github.com/tpn/pdfs/blob/master/Fast%20String%20Correction%20with%20Levenshtein-Automata%20(2002)%20(10.1.1.16.652).pdf
[Lucene]: https://fossies.org/linux/lucene/lucene/core/src/java/org/apache/lucene/util/automaton/Lev1TParametricDescription.java
[moman]: https://gitlab.com/notriddle/moman-rustdoc

User-visible changes
--------------------

I don't expect anybody to notice anything, but it does cause two changes:

- Substring matches, in the middle of a name, only apply if there's three
  or more characters in the search query.
- Levenshtein distance limit now maxes out at two. In the old version,
  the limit was w/3, so you could get looser matches for queries with
  9 or more characters[^1] in them.

[^1]: technically utf-16 code units

Author: notriddle