Accounting for gas consumption

[anorth]

We need a model for accounting for gas consumption. Which operations are accounted for explicitly? How do we account for direct actor code execution that doesn't cross account-ible boundaries like Runtime calls? Explicit or implicit? How good is good enough for initial network launch?

Precise numbers aren't needed yet, but the potential variance in implementation approaches is huge, and the efficiency gains of knowing where we should head significant.

There's the beginning of a discussion in slack here.