Create new Certificate and Vote types and use them for Quorum

[bfish713]

I created mainly 2 new structures: SimpleCertificate and SimpleVote. These implement the most basic vote and certificate pattern.

SimpleVote is some voteable (something we can vote on) data, a signature and a view. It's really important to note that the voteable data is very similar to the old VoteData type except it is no longer an enum. Basically the data member of SimpleVote is the thing that the vote actually votes on (e.g. it is a leaf commitment for the quorum vote) and an implementation of commit. All the voteable implementations of commit add in a tag which commits to which type of vote it is. This prevents forming a QC with different vote types over the same data. (one example would be if we had no votes, both a yes and no vote would be over the same leaf commitment, but the votes would commit to different values).

SimpleCertificate is the basic implementation of a certificate. Like SimpleVote we should be able to use it for all our certificate types (threshould() is the only thing that differs). A certificate is over some voteable data, and commitment to that data.

For votes I created type alias for all the vote types so other code doesn't need to specify the correct Voteable type everywhere in the code.

For certificates I only created the Quorum type because not all the types will use the implementation of threshold in SimpleCertificate I want to follow up to make this function more generic to prevent copypasta of the rest of the Certificate1 trait impl.

I also only implemented the genesis function for the Quorum certificate.

Closes #1965

[elliedavidson]

This really is SO much cleaner than what we had before!

[shenkeyao]

Thanks for making these changes!

[shenkeyao]

LGTM!

[elliedavidson]

Sorry to be a nit, but a minor change request.

[shenkeyao]

LGTM