extend `TypeName` with slice type

[nikomatsakis]

As part of #368, we need to extend chalk's TypeName to model the built-in [T] type. We can add a variant Slice.

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 slices have one generic type parameter.

You would need to

• add Slice to TypeName
• extend parser to accept [T] type
• Update the current state table to include [T], though slices don't I think require any special handling (they are not Sized, and their other impls live in the standard library).

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

[nathanwhit]

@rustbot claim