Publicise types/add #[allow(visible_private_types)] to a variety of places.

There's a lot of these types in the compiler libraries, and a few of the
older or private stdlib ones. Some types are obviously meant to be
public, others not so much.

src/librustdoc/html/render.rs

@@ -99,7 +99,7 @@ pub enum ExternalLocation {
 }
 
 /// Different ways an implementor of a trait can be rendered.
-enum Implementor {
+pub enum Implementor {
     /// Paths are displayed specially by omitting the `impl XX for` cruft
     PathType(clean::Type),
     /// This is the generic representation of a trait implementor, used for