message: Grant users access filtering downstream lanes
diff: #include "caf/downstream.hpp"
  +#include "caf/meta/type_name.hpp"
  +
  namespace caf {
  /// A filtering downstream allows stages to fork into multiple lanes, where
  @@ -45,6 +47,10 @@ public:
  struct lane {
  typename super::queue_type queue;
  typename super::path_ptr_list paths;
  + template <class Inspector>
  + friend typename Inspector::result_type inspect(Inspector& f, lane& x) {
  + return f(meta::type_name("lane"), x.queue, x.paths);
  + }
  };
  /// Identifies a lane inside the downstream. Filters are kept in sorted order
  @@ -110,6 +116,10 @@ public:
  lanes_[std::move(f)].paths.push_back(super::find(x));
  }
  + const lanes_map& lanes() const {
  + return lanes_;
  + }
  +
  private:
  void erase_from_lanes(const strong_actor_ptr& x) {
  for (auto i = lanes_.begin(); i != lanes_.end(); ++i)
  