Add a bus helper similar to "run" to parse messages in parallel

[marco-c]

run currently parses one message after the other:

libmozevent/libmozevent/bus.py

Lines 127 to 134 in 0082aae

	while True:
	message = await self.receive(input_name)
	# Run async or sync methods
	if inspect.iscoroutinefunction(method):
	new_message = await method(message)
	else:
	new_message = method(message)

.

An alternative where messages are parsed concurrently would be useful in some cases (e.g. pulse listening).