This provides an escape hatch for the case where the nMigen platform
code is not flexible enough, and a IO buffer primitive needs to be
instantiated directly.
This markedly differs from oMigen system, which would request
consecutive resources. The difference is deliberate; most resources
are singular, so it does not matter for them, and for resources where
it does matter, which pins are requested should not depend on order
of execution of `platform.request`.
This is necessary because on some platforms, like iCE40, extras
become parameters on an IO primitive, since the constraint file
format is not expressive enough for all of them.
In the simple cases, a Pin record consisting of exactly one field
is equivalent in every way to this single field. In the more complex
case however, it can be used as a record, making the code more robust
such that it works with both bidirectional and unidirectional pins.
