Reducing gas fees

We have a few places where we can potentially reduce gas fees:

1. When opening blocks, we can charge the first time the block was read during the current transaction and charge only a small fee per subsequent access. This will help if we repeatedly call the same actor.
2. We can avoid charging to read blocks we've written.
3. We can likely reduce the cost of invoking actors (see #2057).
4. We may be able to cache "pre-instantiated" wasm module instances, charging only the first time the module is instantiated.
5. We _may_ be able to reduce the cost of system calls, but that will require some tricky benchmarking.

Unfortunately, most of these changes won't affect sector onboarding methods. But they will reduce the cost on-chain operations that interact with multiple actors, making it easier to make complex markets.