Optimized PlainHasher hashing. Trie insertions are >15% faster

[debris]

PlainHasher::write before:

test write ... bench:      18,025 ns/iter (+/- 1,425)

PlainHasher::write after:

test write ... bench:          58 ns/iter (+/- 10)

before:

test trie_insertions_32_mir_1k      ... bench:   3,956,331 ns/iter (+/- 577,529)
test trie_insertions_32_ran_1k      ... bench:   4,009,316 ns/iter (+/- 567,403)
test trie_insertions_random_mid     ... bench:   2,900,765 ns/iter (+/- 283,349)
test trie_insertions_six_high       ... bench:   2,981,152 ns/iter (+/- 331,667)
test trie_insertions_six_low        ... bench:   5,828,811 ns/iter (+/- 593,180)
test trie_insertions_six_mid        ... bench:   3,889,366 ns/iter (+/- 110,769)
test trie_iter                      ... bench:   6,755,724 ns/iter (+/- 1,199,892)

after:

test trie_insertions_32_mir_1k      ... bench:   3,262,684 ns/iter (+/- 118,147)
test trie_insertions_32_ran_1k      ... bench:   3,290,951 ns/iter (+/- 78,756)
test trie_insertions_random_mid     ... bench:   2,654,520 ns/iter (+/- 744,629)
test trie_insertions_six_high       ... bench:   2,492,217 ns/iter (+/- 236,359)
test trie_insertions_six_low        ... bench:   4,989,133 ns/iter (+/- 86,834)
test trie_insertions_six_mid        ... bench:   3,314,200 ns/iter (+/- 84,884)
test trie_iter                      ... bench:   6,749,411 ns/iter (+/- 6,490,631)

[debris]

unrelated benchmark fix

[debris]

I moved PlainHasher to a separate crate. here is the source

[rphmeier]

indentation wrong here: https://github.com/debris/plain_hasher/blob/master/src/lib.rs#L45
BTW, shouldn't it be a paritytech crate? @arkpar and I are also authors of PlainHasher

[debris]

@rphmeier updated readme and moved the repo to https://github.com/paritytech/plain_hasher

I will also add core-devs as owners on crates.io once they fix this bug: rust-lang/crates.io#997

[arkpar]

I don't really see why PlainHasher needs to be on crates.io. It is too parity-specific to be generally useful.

[debris]

@arkpar fixed