Incremental blake2b

[scossu]

Thanks for making this library in C using solid foundations.

The one thing I am missing is a way to compute a blake2b hash incrementally. Monocypher has an interface for that, which shouldn't be hard to integrate in your project: https://monocypher.org/manual/blake2b

I started working on an implementation and when I'm ready I can send a pull request if welcome (I'd rather not keep a fork of my own).