Message interpreter

[aakoshh]

Implement the execution of chain messages using the FVM. Called an interpreter as a homage to https://github.com/ChainSafe/forest/tree/main/vm/interpreter

What's different for us from Forest is:

• We get messages one by one from Tendermint Core, because that part of the ABCI interface hasn't changed. In later stages of ABCI++ rollout, the BeginBlock -> DeliverTx -> EndBlock -> Commit cycle will become FinalizeBlock (plus maybe a Commit).
• The methods will be asynchronous, so they have a chance to do side effects when the message contains CIDs that need to be resolved.

So let's create an interpreter crate like Forest, with a abstract trait to apply messages on some state. The state will depend on the use case: during block execution it will be backed by copy-on-write cache of a block store, and similarly during transaction checking it will have a state that is based on the last executed block, with pending transaction applied but not committed. In Milestone 2 it will also have access to some shared memory through STM where it can coordinate the resolution of CIDs with the other parts of the application.