3x-5x faster hash in nacl-fast.

Get rid of u64 by inlining everything and doing separate calculations on
separate buffers of high and low bits of 64-bit numbers.

Before:

 Benchmarking crypto_hash (1024 bytes)
 1.550 MB/s
 1587.302 ops/s

 Benchmarking crypto_hash (16 KiB)
 2.248 MB/s
 143.885 ops/s

After:

 Benchmarking crypto_hash (1024 bytes)
 3.756 MB/s
 3846.154 ops/s

 Benchmarking crypto_hash (16 KiB)
 11.004 MB/s
 704.225 ops/s