Allow multiple timers in Delay and Timer

[cgm616]

TIMER1 and TIMER2 can now be used for this functionality instead of just
TIMER0. This can be useful for constructing multiple Delays at once (for example, in a multi-app scenario).

[therealprof]

Mentions TIMER0 a couple of times although it is now generic. Also this is a breaking change so we'll need to make a big version jump after adjusting the microbit crate.

[therealprof]

@cgm616 ping

[cgm616]

I've been using these changes a little, and I'm not sure if they're actually advisable. I must be missing something, because I can't figure out how to actually use the Delay with any of the timers. I can't find the exact error message right now, but I can't construct a Delay and then call an actual delay method on it.