Prefer ActorID where possible

[Stebalien]

Currently, we have quite a few places in the code where we:

1. Resolve an actor address into an actor ID.
2. Turn it back into an Address.

Ideally, we'd:

1. Store ActorIDs in state (smaller).
2. Use actor IDs in all syscalls except resolve_address:
   1. Easier to reason about gas usage.
   2. More efficient when the address has already been resolved.

However, this is a significant refactor and likely isn't worth it for now.

[jennijuju]

this may result in different gas charges, including in self_destruct and such.

[anorth]

Replaced by filecoin-project/builtin-actors#56.

We can't do this without a state migration.

[anorth]

Re-open if you want to change the syscall APIs