implement Cancel trait for Timer

[MarcoIeni]

Timer in stm32f4xx-hal crate implements the Cancel trait. You may be interested in doing the same for this crate. Examples:

• https://github.com/stm32-rs/stm32f4xx-hal/blob/master/examples/timer-periph.rs
• https://github.com/stm32-rs/stm32f4xx-hal/blob/master/examples/timer-syst.rs