This commit adds an implementation of machine.Timer backed by the soft
timer mechanism. It allows an arbitrary number of timers with 1ms
resolution, with an associated Python callback. The Python-level API
matches existing ports that have a soft timer, and is used as:
from machine import Timer
t = Timer(freq=10, callback=lambda t:print(t))
...
t = Timer(mode=Timer.ONE_SHOT, period=2000, callback=lambda t:print(t))
...
t.deinit()
