Increase our MSRV to 1.56.

Our support policy says that we can update to any Rust released at least 6 months ago; 1.56 came out on 21 October 2021.

Our main justification for this change is to be able to upgrade to newer versions of our dependencies, including async_executors >= 0.5, aes >= 0.8, and cipher >= 0.4.

This branch also upgrades to edition-2021 and makes corresponding changes. See #208 (closed) for older discussion on that issue.

There's a good case to be made that we shouldn't actually do this on our general-purpose crates (caret and retry-error) since those are potentially usable by other projects; please let me know what you think of that argument.

I don't want us to merge this until we have signoff from @eta and @Diziet, since it's potentially a big change.