The test added for crypto_secure_wipe wipes a buffer with a secret in
it. Only the last byte is kept as it was. The last byte is used to check
that the function doesn't write outside the supplied buffer.
This implementation is optimized for a little code and data size, not
for speed. IMO the code is more readable than in the reference
implementation.
The biggest advantage of ChaCha over other stream ciphers is the very
little data usage with only 64 bytes of context, and its good encryption
speed.
Also part of this PR is pseudo-random number generator, that just
returns the keystream of a randomly initialized ChaCha context.
