syntax: Rename some keywords

`CrateRoot` -> `PathRoot`, `::` doesn't necessarily mean crate root now
`SelfValue` -> `SelfLower`, `SelfType` -> `SelfUpper`, both `self` and `Self` can be used in type and value namespaces now

src/librustc/ty/context.rs

@@ -2710,7 +2710,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 
     #[inline]
     pub fn mk_self_type(self) -> Ty<'tcx> {
-        self.mk_ty_param(0, keywords::SelfType.name().as_interned_str())
+        self.mk_ty_param(0, keywords::SelfUpper.name().as_interned_str())
     }
 
     pub fn mk_param_from_def(self, param: &ty::GenericParamDef) -> Kind<'tcx> {