strings: implement a faster single-string Replacer

strings: implement a faster single-string Replacer

The string searching is implemented separately so other functions
may make use of it in the future.

benchmark                        old ns/op    new ns/op    delta
BenchmarkSingleMaxSkipping          125889         2474  -98.03%
BenchmarkSingleLongSuffixFail        16252         1996  -87.72%
BenchmarkSingleMatch                260793       136266  -47.75%

benchmark                         old MB/s     new MB/s  speedup
BenchmarkSingleMaxSkipping           79.43      4041.57   50.88x
BenchmarkSingleLongSuffixFail        61.65       501.81    8.14x
BenchmarkSingleMatch                 57.52       110.08    1.91x

R=nigeltao
CC=golang-dev
https://golang.org/cl/6545049