Tracking #879.
The directions of signals in `Pin` make it convenient to use a pin
signature in a component, such as in:
class LEDDriver(Component):
pins: Out(Signature({"o": Out(1)}))
led_driver = LEDDriver()
connect(led_driver.pins, platform.request("led"))
The `platform.request` call, correspondingly, returns a flipped `Pin`
object.
The main purpose of this change is migrating glasgow from the compat
`TSTriple` (which allows 0 width) to `Pin`. This sort of change would
normally require a RFC, but `Pin` is already slated for
removal/replacement, so that was deemed to be unnecessary.
