Initialize Genesis state

[aakoshh]

Initialize the FVM state tree from the genesis in #345

Examples of this are:

• https://github.com/anorth/fvm-workbench/blob/main/builtin/src/genesis.rs
• https://github.com/filecoin-project/lotus/blob/v1.20.4/chain/gen/genesis/genesis.go

Forest doesn't have an actor-specific intialisation code, because it loads a singe CAR file prepared by Lotus that has all the state exported in just the right format already.

[aakoshh]

One question is what to pass in InitChain::app_state_bytes?

Option 1

We could, as Tendermint suggests, take the serialized application state

Pros:

• If we go for the first option, we can resume the chain from a snapshot, passing the height to Tendermint.
  Cons:
• In this case we would need a CLI command to take the Genesis and turn it into a CAR file like Lotus. IIRC this is possible with the Rust library, but it's a bit non-intuitive, not clear in the API.
• We also need a way to interrogate the state to produce the list of validators and their power, which we currently don't have an actor to put into.

Option 2

We can take the serialized Genesis type and fill the state in the app.

Cons:

• If this is our only way to start, we'd have to export snapshots in this format.
  Pros:
• We can delay creating an actor to hold the validator state, because the validators are right there in Genesis, we can return them to Tendermint; in this milestone they are not expected to change.