Use PHF for generated matches

[Manishearth]

This improves performance of language rule fetching significantly. Not as much as I'd hoped, but 5x isn't too bad.

Instead of a giant match (which involves a lot of string comparisons), we use phf-map, which at compile time generates an efficient hash function that we can use with the statically generated map for fast lookup.

# before changes
test bench_create ... bench:         604 ns/iter (+/- 73)
test bench_total  ... bench:         943 ns/iter (+/- 159)


# after changes
test bench_create ... bench:         169 ns/iter (+/- 73)
test bench_total  ... bench:         819 ns/iter (+/- 235)

[Manishearth]

r? @unclenachoduh cc @zbraniecki

[zbraniecki]

This will require cargo regenerate-fixtures and cargo regenerate-fixtures-within - https://github.com/unclenachoduh/pluralrules/blob/master/make_pluralrules/.cargo/config

[zbraniecki]

Contributes to #14

[coveralls]

Pull Request Test Coverage Report for Build 106

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 89.884%

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Totals
Change from base Build 105:	0.03%
Covered Lines:	542
Relevant Lines:	603

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💛 - Coveralls

[unclenachoduh]

Thanks for the PR @Manishearth. I updated the fixtures in #22 so this PR can be closed.