Memory structs could use slight cleanup

[daym]

We aren't using SODIMMs so I punted on them for the time being. That would be EntryId::Memory(PsSodimmDdr4MaxFreq) and does or does not also use the same struct MaxFreqElement as we already have.

Ddr5CaPinMapElementLane could use some special pin type that marks 0xff as "unused".

I've had some Option0xff<T> user-visible type before and it really wasn't nice to use.

I wonder whether we can have some official special Option where we can choose the representation of the None value.