benchmark                      old ns/op    new ns/op    delta
BenchmarkAtof64Decimal               344           71  -79.22%
BenchmarkAtof64Float                 397           90  -77.15%
BenchmarkAtof64FloatExp              445          241  -45.84%
BenchmarkAtof64Big                   731          324  -55.68%
BenchmarkAtof64RandomBits            761          453  -40.47%
BenchmarkAtof64RandomFloats          690          314  -54.49%

R=dave, rsc
CC=golang-dev, remy
https://golang.org/cl/5988053