Remove AstDeref.

It's a "utility trait to reduce boilerplate" implemented for `P` and
`AstNodeWrapper`, but removing it gives a net reduction of twenty lines
of code. It's also simpler to just implement
`HasNodeId`/`HasAttrs`/`HasTokens` directly on types instead of via
`AstDeref`.

(I decided to make this change when doing some related refactoring and
the error messages involving `AstDeref` and `HasAttrs` were hard to
understand; removing it helped a lot.)

compiler/rustc_ast/src/lib.rs

@@ -46,7 +46,7 @@ pub mod tokenstream;
 pub mod visit;
 
 pub use self::ast::*;
-pub use self::ast_traits::{AstDeref, AstNodeWrapper, HasAttrs, HasNodeId, HasTokens};
+pub use self::ast_traits::{AstNodeWrapper, HasAttrs, HasNodeId, HasTokens};
 
 /// Requirements for a `StableHashingContext` to be used in this crate.
 /// This is a hack to allow using the `HashStable_Generic` derive macro