rustpkg should support semantic versions

[catamorphism]

Metabug: #5677

There's already a SemVer variant in the Version datatype, but I've completely ignored it so far. There are no tests for it. This should change.

[huonw]

Triage: no change, and git grep implies that the SemanticVersion type is never constructed in rustpkg.

[klutzy]

cc #11396. I think this would work well only if all libraries use semver.

[sinistersnare]

rustpkg is dead, should we close this?