Minor improvements and thread linking

These edits:

 * Abstract octets handling more thoroughly
 * Slightly improve initialization performance by avoiding O(N) zeroing for O(1) zeroing
 * Add thread linking support to build against the new ctypes

Thanks for your contribution of the Bigarray-backed code! I would love to merge this into ocaml-sodium once we work out how thread support is working.

When do you anticipate a ctypes 0.2 with bigarray and togglable thread use?

I'll put together a simple benchmark system sometime soon so we can quantify performance improvements from ctypes advancements in this kind of use case.