tag releases and start publishing to `crates.io`

[dicej]

So far, I've been publishing releases on pypi.org without creating corresponding Git tags, and have made no attempt to publish this crate on crates.io. Hence the following items:

• Retroactively create Git tags for all existing releases and make a habit of tagging future releases
• Publish this crate on crates.io

The latter will probably require auto-downloading a compatible cpython source tree since we don't want to include the whole cpython submodule in the tarball.