Move the crypto module into its own crate

[vkomenda]

The crypto module

• has a different commit dynamic to the rest of the hbbft crate,
• is related to the main code through an interface,
• has it's own CI tests that add up to the CI tests of hbbft and don't have to be run on every commit to the rest of hbbft.

Separating it into its own crate can also facilitate the research on other types of cryptography (#54) in hbbft because other crates could be designed to use the same interface.

Crypto-related issues such as #89 would move with the module as well.

[c0gent]

Yes it's a perfect candidate to be separated then. Sounds like it's probably a good idea to move it sooner rather than later.