tracking: use of unstable features

[kaspar030]

We're aiming to build with stable Rust.

Currently, we're using these unstable features:

1. can't do without:

   • [ ] used_with_arg: this is used by linkme for its distributed slices. User facing!
   • [ ] impl_trait_in_assoc_type: required by Embassy. User facing for applications using the task macro

2. could do without:

   • [ ] type_alias_impl_trait => embassy uses this or pre-allocate futures memory. User facing!
   • [ ] const_mut_refs: used by ringbuffer
   • [ ] custom_test_frameworks: will go when we have migrated to defmt-test
   • [ ] doc_auto_cfg: used only when compiling documentation
   • [x] error_in_core: stabilized and removed in https://github.com/future-proof-iot/RIOT-rs/pull/431
   • [ ] fn_traits
   • [x] inline_const: stabilized
   • [ ] naked_functions: used by riot-rs-threads
   • [x] lint_reasons: stabilized and removed in https://github.com/future-proof-iot/RIOT-rs/pull/419/

user facing means, those need to be enabled in application crates b/c they're used by macros or code we provide.

[ROMemories]

The current experimental sensor abstraction also relies on trait_upcasting. We may be able to work around that.