chacha20 - Returning serde feature along with a compile time warning?

[nstilt1]

As per the discussion here (rust-random/rand#1101) I have added a warning whenever serde is enabled. This assumes that people will read compile-time warnings. I have verified that the warning shows up on dependent crates, but I will add that if the dependency is too far upstream, people might not know whether the warning is something to be worried about, so some game devs might raise an issue about why they keep getting this warning. We could add a feature called non_crypto_PRNG or something similar to silence the warning. Just want to show what the changes would look like if we added the feature back and added a simple warning.

[tarcieri]

I would personally prefer to just not have the feature.

[nstilt1]

Alright then.