Proofs

[mappum]

I'm working on simple proof generation/verification in this branch. This will be query proofs rather than state-sync chunk proofs (which will come in a later update).

This will support:

• Querying N keys
• Proving keys/values, or absence of a key
• Creating query workers, which can generate proofs for queries concurrently (for the last committed tree state), even while changes are being made to the tree

[mappum]

These changes are pretty much ready. Although it went smoother than expected so I'll go deeper in writing tests to try to find issues - possibly a fuzz test which queries random keys.