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