refactor: using &str directly instead of S: Borrow<str>.

[filippodebortoli]

• numbers of clone()s decreased
• number of explicit borrow() calls decreased
• we use & a bit more often
• all public methods generating named individuals have a simpler signature

[phillord]

I am not convinced by this one. We do simplify some of the API by moving from a generic type to a concrete one, but that increases the complexity in many other areas (lots of "&").

ForIRI is now Deref<str> so it feels like many of the borrow and clone could go anyway.