add field access with tests

test/cases/class_def/class_function.tst

@@ -1,22 +1,33 @@
 class Pifi
   ofthen(3 , var)
   int ofthen(int n , ref m)
-    44
+    n = n + m.index
+    return n
   end
 end
 -- -- --
-s(:expressions, 
-  s(:class,  :Pifi, 
-    s(:derives,  nil), 
-    s(:call, 
-      s(:name,  :ofthen), 
-      s(:arguments, 
-        s(:int,  3), 
-        s(:name,  :var))), 
-    s(:function,  :int, 
-      s(:name,  :ofthen), 
-      s(:parameters, 
-        s(:field,  :int,  :n), 
-        s(:field,  :ref,  :m)), 
-      s(:expressions, 
-        s(:int,  44)))))
+s(:expressions,
+  s(:class, :Pifi,
+    s(:derives, nil),
+    s(:call,
+      s(:name, :ofthen),
+      s(:arguments,
+        s(:int, 3),
+        s(:name, :var))),
+    s(:function, :int,
+      s(:name, :ofthen),
+      s(:parameters,
+        s(:field, :int, :n),
+        s(:field, :ref, :m)),
+      s(:expressions,
+        s(:assign,
+          s(:name, :n),
+          s(:operator, "+",
+            s(:name, :n),
+            s(:field_access,
+              s(:receiver,
+                s(:name, :m)),
+              s(:field,
+                s(:name, :index))))),
+        s(:return,
+          s(:name, :n))))))