It turns out that TIM1 and TIM8 have their own Capture/Compare
interrupt vector. For all of the other timers, the capture/compare
interrupt vector is the same as the update vector.

So we need to add handlers for these vectors and enable them
when using capture/compare callbacks.

During testing of this, I also found that passing a channel callback
into the channel constructor would not enable interrupts properly.

I tested using:
```
>>> pyb.Timer(1, freq=4).channel(1, pyb.Timer.OC_TOGGLE, callback=lambda t: print('.', end=''))
```
I tested the above with channels 1, 4, and 8