create TypeName for & and &mut

[nikomatsakis]

As part of #368, we need to extend chalk's TypeName to model the built-in & and &mut types. We can add a variant Ref(Mutability), where Mutability is a new type to add to chalk-ir based on rustc's Mutability enum.

You can follow #394 which provides a rough model of what needs to be extended. You probably want to model this on how tuples are handled, a N-ary tuple like (_, _) has two generic type parameters, and & and &mut have two generic parameters too (a lifetime and a type).

You would need to

• add Ref(Mutability) to TypeName
• extend parser to accept &, &mut types
• Update the current state table to include the new types, I don't think it requires any special handling apart from Sized, I think its impls live in the standard library

This issue has been assigned to @nathanwhit via this comment.

[nathanwhit]

@rustbot claim

[jackh726]

I merged the PR a bit too early. &mut references aren't copy/clone. @nathanwhit