PartialVersion parsing support

[Benjscho]

Creating this issue to discuss about for PartialVersion parsing. Currently Version::parse only supports parsing full versions (i.e., with at least a major, minor, and a patch).

Would you be against adding support for PartialVersion parsing, e.g., in the same manner that Cargo handles partial versions? This would involve adding a new struct PartialVersion, that supports parsing incomplete versions, and implementing impl From<semver::PartialVersion> for Version

I'd be happy to submit a PR to add the struct & parsing if there's interest.

[dtolnay]

I am open to it, but I have seen different projects expect different semantics from a "partial version". I would need to be shown that the behavior you want in this crate is widely applicable. For example I am not sure there is consensus about whether a partial version is supposed to represent build metadata.

[Benjscho]

I'd propose following the implementation in Cargo, where if a PartialVersion can be completely parsed as a Version then it can contain build metadata, e.g., 0.1.0+build parses into a PartialVersion which contains build metadata, but 0.1+build would fail with a build metadata error.