create typename for raw pointers

[nikomatsakis]

As part of #368, we need to extend chalk's TypeName to model the built-in *const and *mut types. We can add a variant Raw(Mutability), where Mutability is the same type described in #438.

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 raw pointers have one generic type parameter.

You would need to

• add Raw(Mutability) to TypeName
• extend parser to accept *const, *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