Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
All the AVRs that I've looked at had the same pin/port structure, with
the possible states being input/floating, input/pullup, low, and high
(with the same PORT/DDR registers). The main difference is the number of
available ports and pins. To reduce the amount of code and avoid
duplication (and thus errors) I decided to centralize this, following
the design used by the atmega2560 but while using a trick to save
tracking a few registers.
In the process, I noticed that the Pin.Get() function was incorrect on
the atmega2560 implementation. It is now fixed in the unified code.
Not tested on actual hardware, only on simavr. The main motivation for
adding this chip is to be able to run simulated tests using a much
larger memory space (16kB RAM, 128kB flash) without jumping to the XMega
devices that may not be as well supported by LLVM.
