Remove support for libnacl leaving only libsodium

The reference implementation of NaCl hasn't seen a release in a
long time and libsodium has several additions we require.

Properly handle strings with internal nul

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.

Add nacl.random(N) to generate a random N bytes

pylint