scrypt: 2x faster.

Work on uint32 slices instead of bytes.
Replace usage of Salsa20/8 from salsa package with
the specialized version.

benchmark       old ns/op    new ns/op    delta
BenchmarkKey    266430525    126657130  -52.46%

R=agl
CC=golang-dev
https://golang.org/cl/7139050
2 files changed