@hackage perfecthash0.2.0

This is an immutable hashtable built on cmph.

The motivation is mostly speed, and wren thornton's bytestring-trie seems to be the main competition:

% ./dist/build/benchmark_trie/benchmark_trie

• trie lookup: 1.136ns per iteration / 880403.98 per second.

% ./dist/build/benchmark/benchmark

• perfect lookup: 0.687ns per iteration / 1456455.69 per second.

it also uses less space in the haskell heap, building once and doing the same number of lookups:

PerfectHash: total alloc = 2,525,223,964 bytes (excludes profiling overheads)

Trie: total alloc = 9,806,202,096 bytes (excludes profiling overheads)

although this is not an entirely fair comparison given how much PerfectHash stores on the C side.