Add Timer::stop

[PvdBerg1998]

Please expose a way to stop a timer. Even better would be a way to init a timer without starting it. The pause line, located at https://docs.rs/stm32f4xx-hal/0.5.0/src/stm32f4xx_hal/timer.rs.html#210-231 could be extracted if I'm correct. The stm32f1xx hal (originally by japaric) already does this: https://docs.rs/stm32f1xx-hal/0.3.0/src/stm32f1xx_hal/timer.rs.html#149.

[marcoieni]

I noticed that the Cancel trait exists in the timer API. Maybe for this function we should implement it?

[therealprof]

@MarcoIeni Sounds like a good idea. Want to give it a shot?

[marcoieni]

Yes, I will try to work on this :)

[marcoieni]

A way to stop a timer has been provided by #137 and #148.

Even better would be a way to init a timer without starting it.

Should we do this? Otherwise we can close this issue.