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/parse/lexer/mod.rs

@@ -849,6 +849,7 @@ impl<'a> StringReader<'a> {
           '@' => { self.bump(); return token::AT; }
           '#' => { self.bump(); return token::POUND; }
           '~' => { self.bump(); return token::TILDE; }
+          '?' => { self.bump(); return token::QUESTION; }
           ':' => {
             self.bump();
             if self.curr_is(':') {