Relicensed with permission of David Fischer <djfische@gmail.com>.

* Provides the ability to pass around a single block of data instead
  of needing to handle transfering the nonce seperately.
* Opens up an avenue for automatic nonce handling in the future.
* Enables passing of nonces manually via explicit nonce parameters.

The strings from NaCl can contain nul characters inside the string
and are not nul terminated. Using ffi.buffer with an explict
size will prevent issues here.