message: Simplify/improve to_string for optional<T>
diff: #include "caf/none.hpp"
  #include "caf/unit.hpp"
  #include "caf/config.hpp"
  +#include "caf/deep_to_string.hpp"
  #include "caf/detail/safe_equal.hpp"
  #include "caf/detail/scope_guard.hpp"
  @@ -302,8 +303,8 @@ inspect(Inspector& f, optional<T>& x) {
  /// @relates optional
  template <class T>
  -auto to_string(const optional<T>& x) -> decltype(to_string(*x)) {
  - return x ? "*" + to_string(*x) : "<null>";
  +std::string to_string(const optional<T>& x) {
  + return x ? "*" + deep_to_string(*x) : "none";
  }
  // -- [X.Y.8] comparison with optional ----------------------------------------
  