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_span/src/symbol.rs

@@ -649,6 +649,7 @@ symbols! {
         dropck_parametricity,
         dylib,
         dyn_metadata,
+        dyn_star,
         dyn_trait,
         e,
         edition_macro_pats,