AVR support has been merged into upstream Rust — now what?

[shepmaster]

There's still a number of issues with AVR code generation, but where should we track such work?

[paulocsanz]

I think the first step is opening those issues on the upstream rust repository, and we should then track them with a reference issue (maybe here because that's the repository most people know about), but I think a reference issue on the upstream rust may be enough.

It's very hard to know which issues here are fixed upstream or have different code upstream than here. We should also update the books and docs, but Idk much about the current situation.