Add new leaf type

[ss-es]

This PR:

• Adds new Leaf2, QuorumProposal2, QuorumVote2, QuorumCertificate2 and QuorumData2 types. Note: these types are purely internal at this time.
• Adds helpers functions to convert between the older (i.e. Leaf, QuorumProposal etc) types and the newly added types
• Adds a migrate_consensus function that is invoked on SystemContext::init, which converts from Leaf to Leaf2 in storage.
• Updates internal hotshot events and logic to use the new types exclusively. All incoming and outgoing messages are converted in the network task.

This PR does not:

• There should be no difference in the logic with this PR, and all commitment calculations should remain the same.
• The actual external message switch (i.e. actually sending QuorumProposal2 out, instead of downgrading to QuorumProposal in the network task) on a version upgrade has not yet been implemented -- this will be done once all types are finalized.
• I have not added epoch numbers to the vote data structs; that will be done in a future PR.
• At this time, the Leaf2 and QuorumProposal2 structs are not final and should not be deployed.

Key places to review:

The types/** crate is the source of the changes and should be reviewed most closely.