Add rand_chacha

[dhardy]

This crate is no longer a dependency of rand

[newpavlov]

I think we can just deprecate the crate completely in favor of chacha20 without keeping the old code in this repository.

[dhardy]

We kept rand_isaac and rand_hc here because they are easy enough to maintain. rand_chacha is no different.

[newpavlov]

It could confuse users, since rand_chacha would look like an "official/recommended" rand crate.