Nicer Argon2 API

Nicer Argon2 API

I believe it's hard to do any better.

- One function to rule them all.
- Inputs are all nicely organised.
- There's an easy way to omit the key and additional data.
- Argon2 user code is very clear, though a little verbose.

I believe fusing the "regular" and "extra" inputs together would not be
a good idea, because it would make the common case (no extra inputs)
either more verbose or more confusing than it is right now.

Issue #243