libsyntax: Allow + to separate trait bounds from objects.

RFC #27.

After a snapshot, the old syntax will be removed.

This can break some code that looked like `foo as &Trait:Send`. Now you
will need to write `foo as (&Trait+Send)`.

Closes #12778.

[breaking-change]

src/librustdoc/clean/mod.rs

@@ -60,7 +60,7 @@ impl<T: Clean<U>, U> Clean<VecPerParamSpace<U>> for VecPerParamSpace<T> {
     }
 }
 
-impl<T: Clean<U>, U> Clean<U> for Gc<T> {
+impl<T: 'static + Clean<U>, U> Clean<U> for Gc<T> {
     fn clean(&self) -> U {
         (**self).clean()
     }
@@ -1198,6 +1198,7 @@ impl Clean<Type> for ast::Ty {
             TyClosure(ref c, region) => Closure(box c.clean(), region.clean()),
             TyProc(ref c) => Proc(box c.clean()),
             TyBareFn(ref barefn) => BareFunction(box barefn.clean()),
+            TyParen(ref ty) => ty.clean(),
             TyBot => Bottom,
             ref x => fail!("Unimplemented type {:?}", x),
         }