An IdentUnique is an identifier with a corresponding span from which it was derived.
Its hash and equality implementation takes the full span into account, meaning that identifiers
are considered unique if they originate from different files.
An Ident is an identifier with a corresponding span from which it was derived.
It relies on a custom implementation of Hash which only looks at its textual name
representation, so that namespacing isn’t reliant on the span itself, which will
often be different.