Rearrange rand later.

[RCasatta]

    I think for now let's just go with this, since it's blocking our move to `rand 0.8`. We can rearrange things later.

Originally posted by @apoelstra in #148 (comment)

What do we want to do?

• Reintroduce rand again and remove CrappyRng
• Move examples to tests/
• Something else

[apoelstra]

I think we should actually reintroduce rand-chacha as a dev-dependency and use that rather than CrappyRng.

Alternately we could expose our own chacha impl from secp-zkp, but that would be kinda involved and I don't know if we want to commit to doing that.

[RCasatta]

I was starting to do this, however, I have a doubt.

let mut rng = rand_chacha::ChaCha20Rng::seed_from_u64(0); 

is probably more dangerous than

let mut rng = CrappyRng::new(core::num::NonZeroU64::new(1).unwrap());

In the first case, you may think to go into production with

let mut rng = rand_chacha::ChaCha20Rng::seed_from_u64(random());  

Which is not secure because the input is only 64 bits.

while I don't think you go into production with CrappyRng

[apoelstra]

Yes, we definitely don't want to use seed_from_u64. We could use from_seed([0; 32]) though, with a comment saying that zero is not a reasonable seed.