give labels an integer that will end up being the position at runtime

Since integers are first class objects, we need to use an integer object
as the return address. The actual address can not be stored in an
instance variable since it is not an object.
The address is unique to the label and never changes after positioning
(using the int is next up)

lib/mom/instruction/basic_values.rb

@@ -11,7 +11,8 @@ module Mom
       @value = value
     end
     def to_parfait(compiler)
-      value = Parfait::Integer.new(@value)
+      value = Parfait.object_space.get_integer
+      value.set_value(@value)
       compiler.add_constant(value)
       value
     end