Faster hash for short inputs

Instead of encoding/decoding state twice from bytes to ints, use int
array for state, and then encode it at the end, as it's normally done.

----
Before:

 Benchmarking crypto_hash (1024 bytes)
 3.488 MB/s
 3571.429 ops/s

After:

 Benchmarking crypto_hash (1024 bytes)
 5.425 MB/s
 5555.556 ops/s