Decouple pin direction overrides, XDRs, etc from request()

[whitequark]

Right now, platform.request() must include every parameter with which the IO buffers will be instantiated. This is unfortunate, because the act of calling platform.request() with a specific name is firmly in the domain of user code, but everything else is just as firmly in the domain of library (peripheral) code.

@jeanthom has hit this in Gram (#413). @ktemkin also hit this in LUNA. It is certain that most advanced peripherals (XDR>0, anything with IOB instances) will also hit this. We should do something about it.