Consider supporting stable Rust

[qwandor]

This crate currently requires nightly Rust, which as far as I can see is just because of the tock-registers crate using #![feature(const_fn_trait_bound)]. I'd like to be able to use at least some parts of the functionality provided by cortex-a on a stable toolchain. Would it be possible to have the parts that require nightly behind a feature, so the other parts can be used on stable?

[qwandor]

Oh I'm wrong, it also uses #![feature(core_intrinsics)] and #![feature(custom_inner_attributes)].