convert the remaining ast record types into structs

These are: region,arg,fn_decl,method,_mod,foreign_mod,
variant_arg,enum_def_,variant_,trait_ref.

src/libsyntax/ast_util.rs

@@ -443,8 +443,11 @@ fn operator_prec(op: ast::binop) -> uint {
 fn dtor_dec() -> fn_decl {
     let nil_t = @ast::Ty { id: 0, node: ty_nil, span: dummy_sp() };
     // dtor has no args
-    {inputs: ~[],
-     output: nil_t, cf: return_val}
+    ast::fn_decl {
+        inputs: ~[],
+        output: nil_t,
+        cf: return_val,
+    }
 }
 
 // ______________________________________________________________________