Contract creator pays for all execution in the ZCFVat

[katelynsills]

The previous issues in #3294 changed the API of Zoe such that fees can be easily charged.

Now it's time to charge for all of the execution that occurs in the ZCFVat, and hook up to the kernel-level meter. This requires connecting the feePurse (which the contract creator supplies when the contract creator calls E(zoe).startInstance) to the meter refill process, and ensuring that the correct fees are charged when the meter is refilled. If the meter cannot be refilled, the ZCFVat should be terminated.

Tests

Unit tests of the meter refill process with a mocked meter.
Swingset tests of the real kernel meter ensuring that when the notification threshold is reached, the meter is refilled.

[katelynsills]

Closed in #3319