message: Expose handler type
diff: @@ -48,6 +48,8 @@ public:
  using unique_pointer = mailbox_element_ptr;
  + using handler_type = std::unique_ptr<inbound_path>;
  +
  static task_size_type task_size(const mailbox_element& x) noexcept;
  // -- constructors, destructors, and assignment operators ------------------
  @@ -69,7 +71,7 @@ public:
  // -- member variables -----------------------------------------------------
  - std::unique_ptr<inbound_path> handler;
  + handler_type handler;
  };
  // -- member types -----------------------------------------------------------
  