strings: implement a faster byte->byte Replacer
When all old & new string values are single bytes,
byteReplacer is now used, instead of the generic
algorithm.
BenchmarkGenericMatch 10000 102519 ns/op
BenchmarkByteByteMatch
1000000 2178 ns/op
fast path, when nothing matches:
BenchmarkByteByteNoMatch
1000000 1109 ns/op
comparisons to multiple Replace calls:
BenchmarkByteByteReplaces 100000 16164 ns/op
comparison to strings.Map:
BenchmarkByteByteMap 500000 5454 ns/op
R=rsc
CC=golang-dev
https://golang.org/cl/
5175050