Event-logged vats

[MostAwesomeDude]

Vats should compute I/O effects by stashing the effect in an "event log", a simple FIFO queue which can be run in-between turns to perform I/O.

This is necessary as part of the leadup to Typhon running vats in parallel with a threadpool.

[MostAwesomeDude]

We should ensure that this design:

• Doesn't block referentially-transparent vats
• Can handle non-libuv events

I'm imagining a class for events:

class Event(object):
    pass

To create a new kind of event, just subclass. Sending an event to a vat should be easy:

event = MyCoolEvent(my, cool, params)
p = currentVat.get().queueEvent(event)

Returning a promise which can be wielded normally.