Use switch_hal rather than manually working with active-low

[chrysn]

Issue status: Mostly a note-to-self; if you like the proposed resolution I can put it into a PR at some later point.

The way the active-low and active-high semantics are packed into the blinq constructor is convenient for typical cases, but falls short when a board already provides the active-high/active-low abstraction through switch-hal. (In particular, for RIOT I currently provide fake pins with fake active-high logic, but I'd rather deprecate that in favor of exposing things at the right semantic level).

Concrete suggestion for blinq:

• Add switch_hal as a dependency
• Implement blinq in terms of switches, moving the constructor to build a switch from the arguments.
  • This is a bit harder than it seems, because in a switch the level logic is in type state whereas here it's in runtime state. (This could need an own implementation of OutputSwitch that uses runtime polarity, but that's just a few lines).
  • This would be a breaking change as Blinq's G type changes (from pin to switch) -- one of these "yes it's a breaking release, but unless you fixed that type somewhere you're good to upgrade" changes.
• Provide a new_from_switch constructor that takes an OutputSwitch (with its on/off semantics), rather than an OutputPin (with its high-low semantics) and a boolean

Alternative: HALs keep providing switches with extra GPIO versions, "It's not really a GPIO, but if some API needs it, use it as OutputPin where it simulates active-high logic". (Which may well be viable).