Ref: #1444

Mentioned on http://forum.espruino.com/conversations/370215/ but also has been an issue for a while when using digitalPulse with arrays of pulses.

On nRF52 we use the 32kHz oscillator for system time, but this means that even though we use the Util Timer which is higher res, when it tries to realign itself with system time, timings can get messed up. I think this isn't helped by tasks being inserted into the timer using an absolute time rather than just a delay.

... so we probably need to adjust util timer functions to take just a delay (and maybe a timer task to set that delay from?), and then modify everything that uses it...