Use perfect cuckoo hash, to avoid binary search.
Define Atom bits as offset+len in long string instead
of enumeration, to avoid string headers.

Before: 1909 string bytes + 6060 tables = 7969 total data
After: 1406 string bytes + 2048 tables = 3454 total data

benchmark          old ns/op    new ns/op    delta
BenchmarkLookup        83878        64681  -22.89%

R=nigeltao, r
CC=golang-dev
https://golang.org/cl/6262051