This uses a similar approach to ComputerThread: executors store how long
they've spent executing tasks. We then use that time to prioritise
executors.
One should note that we use the current runtime at the point of adding
to the queue - external tasks will not contribute towards it until a
later execution.
This effectively acts as a public interface to canExecuteExternal() and
consumeTime(). It's hopefully sufficiently general that we can mess
around with the backend as much as we like in the future.
One thing to note here is that this is based on a polling API, as it's
largely intended for people running work every tick. It would be
possible to adapt this with callbacks for when work is available,
etc..., but that was not needed immediately.
This also removes IComputerOwned, as Plethora no longer needs it.
