Add crypto_secretstream_xchacha20poly1305 API

[dspearson]

I'm looking to use the secretstream API as documented here. Is there any intention to support this?

[dspearson]

It looks like this new API uses structs, namely crypto_secretstream_xchacha20poly1305_state. My experience with jnr-ffi isn't solid enough to know how to approach this unfortunately.

[dspearson]

I did attempt to add bindings, but hit a roadblock:

More than one matching method found: crypto_secretstream_xchacha20poly1305_init_push

My lack of knowledge of jnr-ffi probably renders further work on this useless.

[dspearson]

While working on implementing the kx bindings, I also hit this same issue. It appears to be related to the use of call!, as using the method directly on the sodium object worked in the kx case.