Session/Message API improvements

[fjarri]

• Rename CombinedMessage to MessageBundle
• Expose MessageBundle::session_id(), round(), is_echo(); fixes #123
• SessionId is now public

TODO:

• Take SessionId instead of shared_randomness in Session::new()
• Deal with #118 somehow? Add a broadcast_only() -> Option<SignedMessage>?

[fjarri]

Deleted by mistake.