If user has initiated a delayed event (either with call_in or call_every),
user might need to know how much time is left until the event is
due to be dispatched.
Added time_left() function can be used to get the remaining time.
Added mbed-events from https://github.com/ARMMbed/mbed-events. Changes
from upstream:
- the whole code is licensed under the Apache license. Sources and
headers were updates with this information.
- removed the porting layers for Windows and FreeRTOS and the references
to these porting layers in equeue_platform.h.
- moved the TESTS directory in mbed-events to the TESTS directory of
mbed-os.