`chacha20`: RNG is gated behind `legacy` feature + `rng` feature

[nstilt1]

I had to navigate through some internal dependency conflicts with my library I'm working on, and once I got through them, I noticed that I can't use the RNG because it's gated behind the legacy feature. Well, I can use it by using that feature, but it could be done with only one feature and without the extra cipher dependency. This can be fixed by setting line 62 of variants.rs to #[cfg(any(feature = "legacy", feature = "rng"))]

Also, if XChaCha is supposed to be using the legacy/DJB variant, then we'll need to add the xchacha feature to this cfg

[tarcieri]

Also, if XChaCha is supposed to be using the legacy/DJB variant, then we'll need to add the xchacha feature to this cfg

There are two different variants of XChaCha, one based on the DJB construction, and one based on the IETF construction.

We definitely want to keep the existing implementation.

[nstilt1]

This is the error I'm encountering when trying to compile my library using only the rng feature for chacha20:

error[E0432]: unresolved import `crate::variants::Legacy`
  --> /home/coding/.cargo/git/checkouts/stream-ciphers-beca7f5cd8deeea7/9a7c28d/chacha20/src/rng.rs:24:16
   |
24 |     variants::{Legacy, Variant},
   |                ^^^^^^ no `Legacy` in `variants`
   |
note: found an item that was configured out
  --> /home/coding/.cargo/git/checkouts/stream-ciphers-beca7f5cd8deeea7/9a7c28d/chacha20/src/variants.rs:57:10
   |
57 | pub enum Legacy {}
   |          ^^^^^^
note: the item is gated behind the `legacy` feature
  --> /home/coding/.cargo/git/checkouts/stream-ciphers-beca7f5cd8deeea7/9a7c28d/chacha20/src/variants.rs:56:7
   |
56 | #[cfg(feature = "legacy")]
   |       ^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0432`.
error: could not compile `chacha20` (lib) due to 1 previous error

It should be fixable with #455