Up to 2x faster hash.Hash version.

Inlined rounds, made padding faster.
The shorter the message, the greater the speedup.

benchmark           old ns/op    new ns/op    delta
BenchmarkFull8            224          100  -55.36%
BenchmarkFull16           242          115  -52.48%
BenchmarkFull40           243          117  -51.85%
BenchmarkFull64           347          213  -38.62%
BenchmarkFull128          347          219  -36.89%
BenchmarkFull1K          2299         2042  -11.18%
BenchmarkFull8K         17382        15984   -8.04%

benchmark            old MB/s     new MB/s  speedup
BenchmarkFull8          35.64        79.43    2.23x
BenchmarkFull16         65.86       138.80    2.11x
BenchmarkFull40         98.42       203.96    2.07x
BenchmarkFull64        184.31       299.72    1.63x
BenchmarkFull128       368.14       582.74    1.58x
BenchmarkFull1K        445.40       501.46    1.13x
BenchmarkFull8K        471.29       512.51    1.09x