Add a timer facility to wasi-clocks.

[sunfishcode]

Add a timer facility that allows one to create timers that start at a given time and count down to zero, on a given clock.

The expectation is that this can be used in combination with polling to implement more flexible timeouts.

[codefromthecrypt]

what happens if this is called twice on the same clock? does it invalidate the prior or make a nested timer?

[sunfishcode]

For anyone following along, this question is answered in #13.