for checking for page boundary.  Also avoid boundary check
when >=16 bytes are hashed.

benchmark                        old ns/op    new ns/op    delta
BenchmarkHashStringSpeed                23           22   -0.43%
BenchmarkHashBytesSpeed                 44           42   -3.61%
BenchmarkHashStringArraySpeed           71           68   -4.05%

R=iant, khr
CC=gobot, golang-dev, google
https://golang.org/cl/9123046