This allows for more flexibility in the future. While at it, do note
that the RFC and draft implementations are followed. In particular, in
XChaCha20, hardcode the high word of the counter to 0.
This commit breaks ChaCha20-Poly1305. It'll be fixed in a subsequent
commit.
stash
They both will only alloc and free the internal *_ctx structs. Get rid
of the void * argument for new and only pass arg to *_free instead of
the whole lc_*_ctx struct.
