Utility Timer native functions

[gfwilliams]

The utility timer (used for digitalPulse and Waveform) has been able to do PWM for a while, but it's never been exposed.

For this, I'd like to sort out the contents of jstimer.c - perhaps giving each timer a unique ID, and having one set of functions to access them (currently we have one set for digitalPulse, another for Waveform).

We could then execute native functions on timer IRQs. This would be massively useful for stuff like scanning out displays. You can hack it up on 1v72 by executing code on an IRQ using setWatch, and then physically joining the pin you're watching to a PWM pin - but it's not ideal.

Currently we can't create JsVars from within an IRQ while still remaining stable, so sadly this isn't as useful as it could be. Once that gets fixed it'd be great though.

[gfwilliams]

Just to add, this is used internally. For instance the microbit display uses it. It'd still be nice to expose this to JS.