Missing tags

[nirvdrum]

rustc_version is currently at 0.2.3, but the last tagged release was 0.1.7. Not having tags makes it hard to see what's changed, as there's also no changelog. Fortunately the project is fairly small, so examining all commits is doable. But I think it'd be helpful if all the releases were tagged.

[djc]

I've backfilled all of the tags since 0.2.0.