Keypair generation now ffi-ized

Which is nice.
Add Crypto::Util.zeros

A lot functions require specific buffers of zeros, so make a function to
provide them!
Define a wrap_function macro

This makes calling functions saner, both in terms of length and also in
returning true/false when they work or fail.
Finish conversion to FFI

All functions previous implemented now working via FFI.
Remove the C-ext