Const Container access overloads now return const_element_type
* they were returning element_type::value_type instances which boiled down to instantiating const_element_type and casting it into its value_type anyway
* this was implemented back when BinaryMapping did not enforce constness through its whole stack
* removed element_value_type typedef as one is better served using element_type's value_type typedef
* modifed ConstContainer test case accordingly