Update to rand 0.7

[clintfred]

It would be nice to keep up with new rand versions.

The major hurdle to upgrading is that we have dependencies that don't want to upgrade.

Perhaps we can import two different versions of rand and re-work things so that recrypt holds a separate RNG for ed25519? It would take some experimentation to see if this is possible.

[ernieturner]

Work for this should be an investigation into using multiple different versions of Rand in our project.

[burdges]

I implement a wrapper type shim for this in https://github.com/w3f/schnorrkel/commits/schizomerlin see also rust-random/rand#819 (comment)

[clintfred]

We could upgrade to rand 0.7 is now easier as https://crates.io/crates/ed25519-dalek has released a pre3 that uses rand 0.7.