Implement crypto_stream_xchacha20

[Skycoder42]

I'm interested in non-authenticated XChaCha20, for use with SIV.

@timshadel

This issue was extracted from #5, as the remaining tasks of that issue have all been completed with Version 2.0.0.

[elliotsayes]

Hey interested in this (and regular ChaCha20). My use case is pure encryption as I already have Authentication at another layer in the protocol. What would it involve to complete this?