extmod/uasyncio: Add global exception handling methods.
extmod/uasyncio: Add global exception handling methods.

This commit adds support for global exception handling in uasyncio
according to the CPython error handling:
https://docs.python.org/3/library/asyncio-eventloop.html#error-handling-api

This allows a program to receive exceptions from detached tasks and log
them to an appropriate location, instead of them being printed to the REPL.

The implementation preallocates a context dictionary so in case of an
exception there shouldn't be any RAM allocation.

The approach here is compatible with CPython except that in CPython the
exception handler is called once the task that threw an uncaught exception
is freed, whereas in MicroPython the exception handler is called
immediately when the exception is thrown.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython