Add function for generating keypair from a seed

I ended up using the libsodium version of this function, added [here](https://github.com/jedisct1/libsodium/commit/a821eae055712d1ebda6386ffb1295f26ef2555a).

A few comments:

1. I added this in two commits, one for the underlying C function, and one for the bindings. Let me know if you want me to squash these into one.
2. I didn't add a test because... well, I'm not really sure how to test this (also I can't run the tests at the moment, but the other two pull requests open at the moment fix this). If you can think of something, I'd be happy to try implement it.