Use portable atomic, remove explicit/redundant feature declaration & move to edition 2021

[ithinuel]

No description provided.

[mfp20]

Please merge this pull request to enable use for AVR and other archs. It externalizes the management of missing atomics to specialized crate (portable-atomic).

[Sympatron]

I would keep the defmt_0_3 feature, because defmt v1 will be released soon and can be supported simultaneously that way.

@jamesmunns: Is bbqueue still maintained or should it be forked for future development?

[jamesmunns]

@Sympatron I'm not actively maintaining it at the moment, I've added @ithinuel as a maintainer, but I think they are busy as well.

If you'd like to be added as a maintainer, I'd be happy to add you.

[Sympatron]

That would be nice. I would like to help maintain bbqueue if you two are short on time.

[jamesmunns]

@Sympatron sent you an invite! Feel free to merge PRs as you see fit. Feel free to ping me if you want to coordinate a release.

[Sympatron]

@ithinuel Are you ok with me making changes to this PR to resolve conflicts? And as I said, I would like to keep the defmt_0_3 feature. Or do you want to do it yourself?