semver crate parsing feature parity with node-semver

Currently, there's a number of open issues in the semver crate that involve bringing it to feature parity with node-semver. We need this because of the sorts of things we'll run into in the wild. This issue is a tracking issue for that work -- it's likely we'll need to do it ourselves if we want it done because dev on the crate seems stalled.

Off the top of my head, these are the missing features:

1. "loose" parsing, which involves handling garbage in semver strings. Because it's actually needed. This is an extra option in node-semver but it's used throughout NPM and the ecosystem has come to rely on it.
2. Space-separated range specifiers. This is supported by semver-parser but semver hasn't been updated to use it.
3. Or (||)-separated ranges
4. Other stuff?