uasyncio would benefit from a function to check for event loop instance

[peterhinch]

I appreciate that this is unlikely to be a priority but I thought it worth recording for future reference. There is scope for a subtle bug to occur in user code where a contributed module is used:

import uasyncio as asyncio
import some_module
bar = some_module.Bar()  # Constructor calls get_event_loop()
# and renders these args inoperative
loop = asyncio.get_event_loop(runq_len=40, waitq_len=40)

I can envisage this puzzling users unfamiliar with the code of uasyncio and/or some_module. It could be avoided (with a trivial got_event_loop() function) if the class could test for instantiation.
uasyncio.core.py:

def got_event_loop():
    return _event_loop is not None

In some_module:

class Foo():
    def __init__(self):
        if asyncio.got_event_loop():
            loop = asyncio.get_event_loop()
            loop.create_task(self._run())
        else:
            raise OSError('Foo class requires an event loop instance')