gpio: refactor PWM to be context based

[maruel]

Users forget to stop the PWM at the end of the program, causing user reports like #286 and #363.

We must design an API that makes it clear that the PWM blocking operation must be canceled before the termination of the program.

[maruel]

Discussion about this change is done in https://docs.google.com/document/d/1vQdZdoOMaIan7dKwcAzqHbfM_LCnnSMUQvqypRkjohM.

[maruel]

Moved to periph/conn#3.