Up to 2x faster hash, again.

More hash optimizations: use lots of variables instead of buffers to
keep state in hashblocks and inline loops.

Before:

 Benchmarking crypto_hash (1024 bytes)
 5.425 MB/s
 5555.556 ops/s

 Benchmarking crypto_hash (16 KiB)
 11.161 MB/s
 714.286 ops/s

After:

 Benchmarking crypto_hash (1024 bytes)
 6.510 MB/s
 6666.667 ops/s

 Benchmarking crypto_hash (16 KiB)
 21.404 MB/s
 1369.863 ops/s