fix field default values

test/cases/class_def/class_derived.tst

@@ -9,9 +9,7 @@ end
 s(:expressions,
   s(:class, :Foo,
     s(:derives, :Object),
-    s(:name, :int),
-    s(:assign,
-      s(:name, :field),
+    s(:field_def, :int, :field,
       s(:int, 3)),
     s(:function, :int,
       s(:name, :func),

test/cases/class_def/class_ops.tst

@@ -13,9 +13,7 @@ s(:expressions,
       s(:parameters,
         s(:parameter, :int, :x)),
       s(:expressions,
-        s(:name, :int),
-        s(:assign,
-          s(:name, :abba),
+        s(:field_def, :int, :abba,
           s(:int, 5)),
         s(:operator, "+",
           s(:name, :abba),