we'll take static ownership of an entire pin (not just a limited
reference), otherwise we cannot at all guarantee that the pin will not
be reused for something else while still in use. in theory we could
limit the liftime of this use, but that would require attaching
lifetimes to ClockConfig (and subsequently the main config), passing
those through init(), and returning an object that undoes the gpin
configuration on drop. that's a lot unnecessary support code while we
don't have runtime clock reconfig.
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