Testing shows that for frequencies which the esp8266 can handle -- up to
about 1kHz -- `machine.time_pulse_us()` now gives more accurate results.
Prior to this commit it would measure on average about 1us lower, but now
the average is much closer to the true value. For example a pulse that is
1000us long, it would measure between 998 and 1000us. Now it measures
between 999us and 1001us.
Signed-off-by: Damien George <damien@micropython.org>
398da22492