RFC: Add statically checked macro for safe struct Peripherals creation

[mbuesch]

The Peripherals::take() function has two disadvantages:

• It uses one byte of precious RAM for runtime memory safety checking.
• It uses a couple of bytes of precious program memory for establishing a critical section.

This change uses the linker to statically ensure that only one Peripherals instance is constructed globally.
It introduces a new macro to avr_device that is used just like:

let dp = avr_device::peripherals!(atmega328p);

In case of multiple macro invocations, the linker will abort with

multiple definition of `__ERROR__avr_device__peripherals__macro_must_only_be_used_once__'

TODO:

• For actually not wasting the one byte of memory, the DEVICE_PERIPHERALS access has to be removed from the generated Peripherals::steal() function.
• In order to be sound, all Peripherals::take() function have to be removed from the generated code.

[mbuesch]

As we need support from the svd2rust tool to properly implement this, the macro should probably also be moved into the generated code.
The svd2rust could get a new option to select between static and dynamic peripherals struct checking. avr-device could then always request generation of the static macro.

[Rahix]

Hi, sorry, I completely lost track of this. I agree with you, I think this should be discussed with the svd2rust project and integrated there.