syntax: deprecate #[auto_{en,de}code] in favour of #[deriving({En,De}codable)].

Replace all instances of #[auto_*code] with the appropriate #[deriving] attribute
and remove the majority of the actual code, leaving stubs to refer the user to
the new syntax.

src/libsyntax/ast_util.rs

@@ -370,8 +370,7 @@ pub fn empty_generics() -> Generics {
 // ______________________________________________________________________
 // Enumerating the IDs which appear in an AST
 
-#[auto_encode]
-#[auto_decode]
+#[deriving(Encodable, Decodable)]
 pub struct id_range {
     min: node_id,
     max: node_id,