&str can be turned into @~str on demand, using to_owned(), so for
strings, we can create a specialized interner that accepts &str for
intern() and find() but stores and returns @~str.
This adds support for static methods, and arguments of most types, traits with
type parameters, methods with type parameters (and lifetimes for both), as well
as making the code more robust to support deriving on types with lifetimes (i.e.
'self).
