Export nacl.lowlevel.crypto_core_hsalsa20.
Add to changelog.
Fix benchmark.
Add hash benchmark for 16 KiB.
Build.
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
Implement nacl.sign.keyPair.fromSeed.

Allows deriving a new sign key pair deterministically from a 32-byte
seed. Introduces a tiny change to the low-level crypto_sign_keypair():
a new argument that indicates that sk already contains seed bytes;
however crypto_sign_keypair() interface behaves exactly the same as
before if this argument is not truthful (e.g. not set).

Closes #37.
Docs: clarify fromSecretKey.