EVM Saving & Reloading Contract State

[Stebalien]

We need to efficiently:

• Load contract state on first access.
• Save contract state before making a new call.
• Reload contract state when returning from a call, if and only if something has changed.

The current runtime abstraction doesn't work for us here, so we'll likely have to extend it. Specifically, we need to, e.g., be able to learn the state "root" without actually reading it.

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Examples:

1. If we call into an actor that doesn't read any slots, it shouldn't load the HAMT.
2. If we make a call from an actor that hasn't written to any slots, we shouldn't write any state before making the call.

[Stebalien]

Note: in terms of reverts, the FVM handles that for us. The FVM keeps state "layers" for every call. If any call "reverts", all effects of that call (recursively) are discarded.

[Stebalien]

Proposed Design

1. Introduce "dirty" tracking on first write and/or nonce change.
2. Wrap rt.send with a function that:
   1. Flushes the current HAMT/state if the state is "dirty", recording the new state root in memory.
   2. Executes a callback.
   3. Gets the new state-root CID from the system (sself::get_root) and compares it with the saved root.
   4. If the state has changed (e.g., due to a recursive call), reload it (discarding the current HAMT and any cache therein).

NOTE: we could exempt method 0 sends, but that's probably not worth it.

[maciejwitowski]

@Stebalien I added this to the checklist based on Quartz milestone assigned.