message: das.Symbol.Effective: Add helpers to change passing method.
diff: @@ -12,6 +12,11 @@ namespace das
  : public Symbol
  {
  public:
  + /** A valued symbol
  + *
  + * @param E The value type.
  + * @param P The type through which the value was passed.
  + */
  template <typename E, typename P>
  struct Effective
  {
  @@ -33,6 +38,11 @@ namespace das
  : value(std::forward<Passing>(v))
  {}
  + using ByConstRef = Effective<E, std::remove_cv_reference_t<P> const&>;
  + using ByRef = Effective<E, std::remove_cv_reference_t<P>&>;
  + using ByRValueRef = Effective<E, std::remove_cv_reference_t<P>&&>;
  + using ByValue = Effective<E, std::remove_cv_reference_t<P>>;
  +
  static inline
  std::string
  name()
  