Allow multiple Proposals and/or one Commit in an MLSPlaintext about mls-protocol

[bifurcation]

On discussion at the interim 2019-01: It may be better to do not just throw a blind combo layer in here, but rather to have more operation types. That way, we can fully think through the implications of each one.

[bifurcation]

Deferred for further discussion on the mailing list.

[bifurcation]

Repurposing this issue for combo moves with regard to Proposals and Commits. We should cover the following cases:

• One or more Proposals with no Commit
• A Commit with no Proposals
• One or more Proposals bundled with a Commit

It seems like something like the following would be appropriate:

struct{
  Proposal proposals<0..2^16-1>;
  optional<Commit> commit;
} Handshake;

... together with a requirement that at least one of them be populated.

[bifurcation]

At the interim:

• Commit must cover all Proposals in the same message
• As a result, if any of the Proposals are invalid, then the whole thing fails

[bifurcation]

Fixed in #242