Remove the tuple based return in the Pwm subsystem

[justacec]

The current implementation of the Pwm subsystem returns a tuple, one entry in the tuple for each channels/pins that were configured. This leads to calls such as

pwm_channels.1.enable();

In this call it is not clear if the 1 is the first, second, third, or fourth channel in the timer. It is dependent on the way upon the pwm method is called.

I propose that the pwm method instead return a fixed structure with the four possible enumerated channels. For channels that are not configured, they could take on the value of NoChannel and channels that are configured can be instances of a PwmChannel.

Once this change is in place, the Pwm trait implementation should be adjusted to do all operations on the configured channels to reduce code duplication.