`rand_core` v0.9 support

[EricZHANG1688]

So with v 0.9.0 for rand and rand_core, will get a error like 'error[E0277]: the trait bound ThreadRng: CryptoRngCore is not satisfied".

[c410-f3r]

With the possible addition of rust-lang/rust#130703 it would be nice to avoid hard-coding rand in public interfaces.

For example, some functions could provide a callback where the user is responsible for providing entropy.

pub fn encrypt(msg: &[u8], cb: impl FnOnce(&mut [u8])) -> Result<Vec<u8>> {
  ...
}

let mut rng = thread_rng();
encrypt(&[1, 2, 3, 4], |slice| {
  rng.fill_bytes(slice);
})?;

[Its-Just-Nans]

Related to RustCrypto/traits#1642

[tarcieri]

@c410-f3r the nice thing about CryptoRng is it provides a type-safe interface for ensuring the supplied RNG is actually safe to use for cryptography. A "just make some bytes" API like that can be easily misused with RNGs which are unsuitable for cryptographic use.

In the next set of releases of the https://github.com/rustcrypto/traits crates, we're also providing (and pushing people towards) a non-parameterized random generation APIs which avoids compatibility problems between crate versions and explicit RNGs: RustCrypto/traits#1371