Improve the release process

[bnaecker]

Releasing new versions of the source is currently an entirely manual process. cargo-release could help, but has several odd interactions with workspaces, such as missing interpolation of version numbers in automated commits. This might be addressed by a small shell script which amends the commit history, or we might prefer to be more explicit about how the version numbers of the crates are bumped in relation to one another.

This issue tracks:

1. Determining how the versions of the crates in the workspace are bumped in relation to one another
2. To the extent possible, using an automated or semi-automated process for adding and pushing commits and publishing the new version(s) on crates.io.

[bnaecker]

cargo-release is not exactly worth its weight at this point. It requires a lot of configuration, and it doesn't play that well with the git history. I have a local branch with a shell script based off this from the Tokio project, which seems to provide the right amount of structure. But in any case, I'm putting this on hold for the moment, to work on other things.