exp/html/atom: faster, hash-based lookup.

exp/html/atom: faster, hash-based lookup.

exp/html/atom benchmark:
benchmark          old ns/op    new ns/op    delta
BenchmarkLookup       199226        80770  -59.46%

exp/html benchmark:
benchmark                      old ns/op    new ns/op    delta
BenchmarkParser                  4864890      4510834   -7.28%
BenchmarkHighLevelTokenizer      2209192      1969684  -10.84%
benchmark                       old MB/s     new MB/s  speedup
BenchmarkParser                    16.07        17.33    1.08x
BenchmarkHighLevelTokenizer        35.38        39.68    1.12x

R=r
CC=golang-dev
https://golang.org/cl/6261054