Add impl Trait and dyn Trait types

- refactor bounds handling in the AST a bit
 - add HIR for bounds
 - add `Ty::Dyn` and `Ty::Opaque` variants and lower `dyn Trait` / `impl Trait`
   syntax to them

crates/ra_syntax/src/ast.rs

@@ -15,7 +15,7 @@ use crate::{
 
 pub use self::{
     expr_extensions::{ArrayExprKind, BinOp, ElseBranch, LiteralKind, PrefixOp},
-    extensions::{FieldKind, PathSegmentKind, SelfParamKind, StructKind},
+    extensions::{FieldKind, PathSegmentKind, SelfParamKind, StructKind, TypeBoundKind},
     generated::*,
     tokens::*,
     traits::*,