machine.time_pulse_us() is intended to provide very fine timing, including
while working with signal bursts, where each transition is tracked in row.
Throwing and handling an exception may take too much time and "signal loss".
So instead, in case of a timeout, just return negative value. Cases of
timeout while waiting for initial signal stabilization, and during actual
timing, are recognized.
The documentation is updated accordingly, and rewritten somewhat to clarify
the function behavior.
machine.POWER_ON is renamed to machine.PWRON_RESET to match other
reset-cause constants that all end in _RESET. The cc3200 port keeps a
legacy definition of POWER_ON for backwards compatibility.
