Bring in line with atsamd-hal GPIO v2
Copied as much as possible. Docs lifted mostly as-is.
Also add sample BSP for the Feather RP2040 in boards/feather_rp2040
May include a few random fixes from currently futile attempt to get doctests working.
It seems like the rom_table_lookup missed one level of
pointer dereferencing.
After comparing it to the working call to reset_usb_boot() in
https://github.com/jannic/rp-microcontroller-rs/blob/master/util/rp2040-panic-usb-boot/src/lib.rs,
I changed the code until it generated basically the same assembly.
With that change, I was able to successfully call rom_data::reset_to_usb_boot()
I still don't like the type RomTableLookupFn, which just returns
some generic T without any checks, and I think rom_table_lookup
should be unsafe. But as none of those are pub, it doesn't matter too
much. So I just made the changes necessary to make the code work.
