Increase salt size to 32 bytes

[defuse]

The salt for HKDF should be 32 bytes, not 16, because for a fixed user-provided key, we want the full AES-256 keyspace to be available. See also:

http://blog.cr.yp.to/20151120-batchattacks.html

[paragonie-scott]

#137

[defuse]

This was done in #137.