py: Implement new sys.atexit feature.

This patch implements a new sys.atexit function which registers a function
that is later executed when the main script ends.  It is configurable via
MICROPY_PY_SYS_ATEXIT, disabled by default.

This is not compliant with CPython, rather it can be used to implement a
CPython compatible "atexit" module if desired (similar to how
sys.print_exception can be used to implement functionality of the
"traceback" module).