tvx simulate: take a raw message, play it against a tipset, and generate a vector

[raulk]

$ tvx simulate --msg <raw_msg> --atop-epoch <height>

1. Instantiate the proxying blockstore.
2. Execute the message against the provided height.
3. Print out the receipt.
4. Perform a statediff to visualise what changed in the state tree.

Future extensions

Create a tipset class vector: --class=tipset --miner=f101.