Reduce headers processed through C2Rust to minimum

[chrysn]

We should use C2Rust only where we needed, not all across the board.

C2Rust only supports C99, no atomics, and who knows what else not – so let's not make C2Rust the default for headers, but just opt in to it.

This is a breaking change for riot-sys' downstreams, but does not (or should not: testing will show, I only did a few samples;will not, before it is merged) break riot-wrappers or known applications.

In parallel to this, we may want to explore

• how to use bindgen to generate non-static versions of functions into a C file, build and link that, and be fine
• whether any casual use of riot_sys::inline:: can (now?) be done away with

Testing

riot-sys' tests are rather minimal; a test PR in riot-wrappers will give us better coverage.