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

tests/rustdoc-js-std/deduplication.js

@@ -5,6 +5,5 @@ const EXPECTED = {
     'others': [
         { 'path': 'std::f32', 'name': 'is_nan' },
         { 'path': 'std::f64', 'name': 'is_nan' },
-        { 'path': 'std::option::Option', 'name': 'is_none' },
     ],
 };

tests/rustdoc-js-std/path-maxeditdistance.js

@@ -3,16 +3,8 @@ const FILTER_CRATE = "std";
 const EXPECTED = [
     {
         query: 'vec::intoiterator',
-        others: [
-            // trait std::iter::IntoIterator is not the first result
-            { 'path': 'std::vec', 'name': 'IntoIter' },
-            { 'path': 'std::vec::Vec', 'name': 'into_iter' },
-            { 'path': 'std::vec::Drain', 'name': 'into_iter' },
-            { 'path': 'std::vec::IntoIter', 'name': 'into_iter' },
-            { 'path': 'std::vec::ExtractIf', 'name': 'into_iter' },
-            { 'path': 'std::vec::Splice', 'name': 'into_iter' },
-            { 'path': 'std::collections::vec_deque::VecDeque', 'name': 'into_iter' },
-        ],
+        // trait std::iter::IntoIterator is not the first result
+        others: [],
     },
     {
         query: 'vec::iter',