Consider interning strings

[sunshowers]

At the moment, we do several clones of strings for package IDs, feature IDs, etc. This isn't great, and ideally a graph would carry with it a pool of strings + internal references to them instead.

I've done some initial exploratory work at https://github.com/sunshowers/cargo-guppy/tree/string-interning, converting over PackageMetadata to have a lifetime parameter.

Related to #37 since interning package IDs will mean returning a custom PackageId<'g> instead of &'g PackageId.