Change DST syntax: type -> Sized?

closes #13367

[breaking-change] Use `Sized?` to indicate a dynamically sized type parameter or trait (used to be `type`). E.g.,

```
trait Tr for Sized? {}

fn foo<Sized? X: Share>(x: X) {}
```

src/libsyntax/ext/deriving/generic/mod.rs

@@ -406,8 +406,8 @@ impl<'a> TraitDef<'a> {
 
             cx.typaram(self.span,
                        ty_param.ident,
-                       ty_param.sized,
                        OwnedSlice::from_vec(bounds),
+                       ty_param.unbound.clone(),
                        None)
         }));
         let trait_generics = Generics {