Fix testDepends in default.nix

Signed-off-by: Austin Seipp <aseipp@pobox.com>
ChaCha/20 support. Closes #10.

This adds a ChaCha20 module for streaming support. There is no SecretBox
counterpart for ChaCha20+Poly1305. It's also not exported from the
top-level interface either.

I wrote this implementation. With a modern GCC, the result is damn fast
while remaining simple and beautiful (approx. 6.41/cpb on an i7-4770 @
3.40GHz).

Signed-off-by: Austin Seipp <aseipp@pobox.com>