[performance] Improve the performance of damerau_levenshtein

[lovasoa]

My tests show the new version is approximately 2.5 times faster than
the old one. Most of the gains come from using hashbrown for the
hashmap.

[lovasoa]

@dguo

Cargo bench before:

test benches::bench_damerau_levenshtein            ... bench:      19,828 ns/iter (+/- 1,693)
test benches::bench_hamming                        ... bench:          61 ns/iter (+/- 19)
test benches::bench_jaro                           ... bench:         958 ns/iter (+/- 147)
test benches::bench_jaro_winkler                   ... bench:         985 ns/iter (+/- 47)
test benches::bench_levenshtein                    ... bench:       1,491 ns/iter (+/- 139)
test benches::bench_levenshtein_on_u8              ... bench:       1,633 ns/iter (+/- 96)
test benches::bench_normalized_damerau_levenshtein ... bench:      20,257 ns/iter (+/- 787)
test benches::bench_normalized_levenshtein         ... bench:       1,516 ns/iter (+/- 162)
test benches::bench_osa_distance                   ... bench:       2,983 ns/iter (+/- 277)

and after:

test benches::bench_damerau_levenshtein            ... bench:       8,208 ns/iter (+/- 774)
test benches::bench_hamming                        ... bench:          72 ns/iter (+/- 8)
test benches::bench_jaro                           ... bench:       1,001 ns/iter (+/- 225)
test benches::bench_jaro_winkler                   ... bench:         994 ns/iter (+/- 85)
test benches::bench_levenshtein                    ... bench:       1,523 ns/iter (+/- 95)
test benches::bench_levenshtein_on_u8              ... bench:       1,680 ns/iter (+/- 233)
test benches::bench_normalized_damerau_levenshtein ... bench:       8,874 ns/iter (+/- 641)
test benches::bench_normalized_levenshtein         ... bench:       1,602 ns/iter (+/- 182)
test benches::bench_osa_distance                   ... bench:       3,095 ns/iter (+/- 144)

[dguo]

Hey, thanks. This looks awesome. My only concern is bumping the required Rust version to 1.31 since it was only released four months ago. Is that a requirement for hashbrown?

[lovasoa]

yes, it's a requirement of hashbrown : https://github.com/Amanieu/hashbrown/blob/master/CHANGELOG.md

[lovasoa]

Many crates depend on rust 1.31, since it marked the beginning of the 2018 edition.

[dguo]

Sounds good. A project that can't upgrade yet for some reason can always use an older version.

I published the change as v0.9.1.