The main task idea is really really wrong!

[goodboy]

After an in depth discussion with @vodik surrounding #11, it's so obvious now that the main kwarg to ActorNursery.start_actor() is just plain stupid and wrong. The idea of a main task is the same as starting an actor and then submitting a single function/task to that actor and then cancelling the actor once it's lone task is complete. So instead of this inverted approach (of making an actor with a main task the lower level API thus severely complicating the internal implementation of Actor._async_main()) we agreed the nursery should instead get a ActorNursery.run_in_actor() method which internally is simply a wrapper around:

async def run_in_actor(self, name: str, main: Function):
    portal = self.start_actor(name, rpc_module_paths=['my_module_with_func'])
    self._main_children.append(portal)
    return await portal.run('my_module_with_func', func_name)
    await portal.cancel_actor()

When the nursery block ends this actor is waited upon for it's final result much like is done implicitly in the core...
Couple notes:

• the Portal will need to learn a completed state that signals the far end task is done (in the case of an async generator this happens when either end closes - i.e. .aclose())
• the completed state variable will likely need to be a trio.Event

Note this also avoids the confusion in how does the main=my_func module in the remote actor get figured out? (hint it doesn't since this was using the multiprocessing.forkserver's internal pickling stuff to make it happen).

This let's us entirely drop the idea of Portal.result() afaik which in turn means we lose all future-like junk just like trio and a bunch of implementation stuff should be severely simplified!

^ Hmm not sure it should get dropped otherwise it gets tricky trying to figure out how you can call run_in_actor() without blocking. Maybe it's ok to look somewhat like the Future returned from loop.run_in_executor() as in asyncio? This also allows for additional portal.run() calls to be made to that same actor while the nursery block is open.