This will allow re-exporting only certain enum variants.

I would like to rename it to `Type::Path`, but then it can't be
re-exported since the name would conflict with the `Path` struct.
Usually enum variants are referred to using their qualified names in
Rust (and parts of rustdoc already do that with `clean::Type`), so this
is also more consistent with the language.

At last! The new name is shorter, simpler, and consistent with
`hir::Ty`.