Use copy rather than a hand rolled loop when moving a partial input
block to the scratch area. This results in a reasonable performance
gain when partial blocks are written.

Benchmarks on Intel(R) Xeon(R) CPU X5650  @ 2.67GHz with Go amd64:

       benchmark               old MB/s     new MB/s  speedup
SHA1   BenchmarkHash8Bytes        18.37        22.80    1.24x
SHA256 BenchmarkHash8Bytes        11.86        13.78    1.16x
SHA512 BenchmarkHash8Bytes         4.51         5.24    1.16x

       benchmark              old ns/op    new ns/op    delta
SHA1   BenchmarkHash8Bytes          435          350  -19.54%
SHA256 BenchmarkHash8Bytes          674          580  -13.95%
SHA512 BenchmarkHash8Bytes         1772         1526  -13.88%

R=agl, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/35840044