Phase out usage of iomem.ld

[mcoffin]

Most of the iomem.ld stuff can be moved in to native rust I think. (ping @farcaller).

We can just have them as rust statics that get changed to &'static references to their actual types. This is what I see for a lot of the newer register code.

Is there a downside to doing this @farcaller? As for #329, this would allow us to completely do away with mcu-specific linker scripts (with the exception of maybe k20).

Also kinda related to #403, as this, as long as it's 0-cost, will help us phase out a huge amount of our extern static usage.

[farcaller]

yep, sounds like a perfect way to tackle it. The missing thing is that the current code for pointer casters doesn't support several registers of one type.