Feat: display a impl that roundtrips.

[Eh2406]

It would be lovely if a VersionReq that has been converted to a SemverPubgrub still has the same display. SemverPubgrub can represent lots of things that VersionReq cannot so the mapping is not 1:1, but as close as we can get the better. I think a SemverPubgrub can be expressed as a list of VersionReq, where SemverPubgrub contains a version if and only if one of the listed VersionReq matches that version. This requires a lot of implementation and testing.