message: Switch to nested namespace syntax
diff: #include "caf/fwd.hpp"
  -namespace caf {
  -namespace mixin {
  +namespace caf::mixin {
  // TODO: legacy API. Deprecate with 0.18, remove with 0.19.
  template <class T>
  struct is_blocking_requester : std::false_type {};
  -} // namespace mixin
  -} // namespace caf
  +} // namespace caf::mixin
  namespace caf {
  