rewrite dynamic-vat device to accomodate async import-bundle

[warner]

The old @agoric/evaluate was synchronous, just like plain eval. This enabled the dynamic-vat creation device to allocate a vat-id, evaluate the new vat's source code, and return the vat-id to the caller, all in a single turn.

The new import-bundle function is async, to make room for more sophisticated module loading/importing mechanisms in the future.

The task is to rewrite the dynamic-vat creation code to accomodate this async function. The vat-id needs to be returned immediately, but we should allow the actual creation of the vat to happen later. We already send an async message back to the caller with a reference to the new vat's root object. The tricky bit is that the device doesn't get to participate in promises (vats do promises, but not devices), so hooking up the completion callback to a place that can do something useful with it will take a bit of work.

[warner]

This landed as part of #1201