Introduce dyn_star feature flag

The primary purpose of this commit is to introduce the
dyn_star flag so we can begin experimenting with implementation.

In order to have something to do in the feature gate test, we also add
parser support for `dyn* Trait` objects. These are currently treated
just like `dyn Trait` objects, but this will change in the future.

Note that for now `dyn* Trait` is experimental syntax to enable
implementing some of the machinery needed for async fn in dyn traits
without fully supporting the feature.

compiler/rustc_ast/src/ast.rs

@@ -2072,6 +2072,7 @@ impl TyKind {
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum TraitObjectSyntax {
     Dyn,
+    DynStar,
     None,
 }