Convert ContainedSpan to ContainedSpan<T>

[JohnnyMorganz]

Converts ContainedSpan to ContainedSpan<T>, and cleans up all the fields to do so.
This PR is quite big, with a lot of busywork, so a few highlights:

• ContainedSpan is changed to ContainedSpan<T>, with a new field inner, and tokens split into start and end
• ContainedSpan now properly implements Visit, VisitMut and Display
• Some visitors which were explicitly declared because they were Enums with a ContainedSpan are now derived automatically
• Derive macros are simplified: node(full_range) and visit(contains) are now removed

A few things to point out which I was not sure about:

• TableConstructor and GenericDeclaration (Luau) now only have one field, should they be left as structs or converted to enums?
• In some places we had 2 separate methods to retrieve the contained tokens (in a ContainedSpan) and the node contained inside. To somewhat make the change easier, I kept these two methods, but the method to retrieve the contained tokens now return a (&TokenReference, &TokenReference) rather than a ContainedSpan. Is it worth keeping these two methods separate, or just creating a new method which returns the ContainedSpan<T>? [Same issue with the corresponding "mutator" methods]

Still TODO:

• Verify tests are still correct
• Update changelog (quite a few breaking changes)
• Check if the public API is still alright to use (will test this in a downstream consumer)

This PR also fixes #161 (supersedes #166)