Now that Rust 2018 is almost upon us, embedded Rust crates can hope to start working on stable Rust. We should update the awesome-embedded-rust list to track which crates do or do not work on stable.

I imagine this being either a badge (but see #32) or a note or a new section.

Hopefully many crate authors will be update their listings themselves, but if not it is also hopefully not too much work to test arbitrary crates to find out. I think ideally our standard for inclusion should be "the project's public CI indicates it builds on stable".