message: Fix deprecated copy warning
  This fixes a -Wdeprecated-copy warning for clang-10: Definition of
  implicit copy assignment operator for 'unit_t' is deprecated because it
  has a user-declared constructor.
diff: @@ -28,13 +28,9 @@ namespace caf {
  /// to enable higher-order abstraction without cluttering code with
  /// exceptions for `void` (which can't be stored, for example).
  struct unit_t : detail::comparable<unit_t> {
  - constexpr unit_t() noexcept {
  - // nop
  - }
  + constexpr unit_t() noexcept = default;
  - constexpr unit_t(const unit_t&) noexcept {
  - // nop
  - }
  + constexpr unit_t(const unit_t&) noexcept = default;
  template <class T>
  explicit constexpr unit_t(T&&) noexcept {
  