ast: Stop using Mod in Crate

Crate root is sufficiently different from `mod` items, at least at syntactic level.

Also remove customization point for "`mod` item or crate root" from AST visitors.

compiler/rustc_builtin_macros/src/proc_macro_harness.rs

@@ -91,7 +91,7 @@ pub fn inject(
     }
 
     let decls = mk_decls(&mut krate, &mut cx, &macros);
-    krate.module.items.push(decls);
+    krate.items.push(decls);
 
     krate
 }