Add bindings for key derivation functions

[hce]

libsodium provides an interface to

(a) Derive keys using a high entropy master key, a sequential index and a context (8 byte string);
(b) Derive a key from a password.

I don't see bindings for them so far.

Would you accept a pull request for this?

[linearray]

I added bindings for (b) in master and I would certainly accept them for (a).