Added pyb.elapsed_millis and pyb.elapsed_micros

tested using:
stmhal: https://github.com/dhylands/upy-examples/blob/master/elapsed.py
teensy: https://github.com/dhylands/upy-examples/blob/master/teensy/elapsed.py
Merge pull request #881 from dhylands/elapsed

Added pyb.elapsed_millis and pyb.elapsed_micros