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
• error_in_core: stabilized and removed in feat(build): bump toolchain to nightly-2024-09-19 #431
• fn_traits
• inline_const: stabilized
• naked_functions: used by riot-rs-threads
• lint_reasons: stabilized and removed in chore(build): bump Rust toolchain #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.