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/test/compile-fail/unsized-enum.rs

@@ -10,5 +10,5 @@
 
 // error-pattern: instantiating a type parameter with an incompatible type
 fn bar<T: Sized>() { }
-fn foo<type T>() { bar::<Option<T>>() }
+fn foo<Sized? T>() { bar::<Option<T>>() }
 fn main() { }