Backport non-breaking features to 0.6.x

[joshlf]

Since 0.7.0 contains a lot of new features, it's taking a while to release. However, a lot of the features we've added in preparation for 0.7.0 aren't breaking changes. It would be good if we could backport those changes to the 0.6 minor version and release them under new 0.6.x releases.

I've created the v0.6.x branch, which so far is up-to-date with main through d05b7be. The next commit (f86bdd1) bumps the MSRV, which means that any commit after that one might be a breaking change, and it would be difficult from inspecting the source code alone to determine whether that's the case (since it would be easy to not notice that the code relies on a feature which was introduced after 0.6.1's MSRV). The remaining steps are:

• Make sure CI works properly when submitting a PR to merge into the v0.6.x branch (all of the CI code was designed and tested for merging into main, so some of it may not work for merging into a different branch)
• For each change after d05b7be which we want to include, submit a PR and make sure that all tests pass in order to ensure that the change is compatible with 0.6.1's MSRV

[joshlf]

Done; the most recent nonbreaking change was backported in #223.