hash: more efficient memory allocation

hash: more efficient memory allocation

Feed append the complete content at once.

BenchmarkAdler32KB       1000000              2534 ns/op         404.05 MB/s
BenchmarkCrc32KB          500000              4757 ns/op         215.26 MB/s
BenchmarkCrc64KB          500000              4769 ns/op         214.70 MB/s
BenchmarkFnv32KB         1000000              2417 ns/op         423.64 MB/s
BenchmarkFnv32aKB        1000000              2408 ns/op         425.23 MB/s
BenchmarkFnv64KB          500000              4262 ns/op         240.21 MB/s
BenchmarkFnv64aKB         500000              4234 ns/op         241.83 MB/s

R=iant, rsc, r, minux.ma
CC=golang-dev
https://golang.org/cl/5937053