message: Fallback to std::span if available
diff: #pragma once
  +#if __cplusplus > 201703L
  +
  +#include <span>
  +
  +namespace fizzy
  +{
  +template <typename T>
  +using span = std::span<T>;
  +}
  +
  +#else
  +
  #include <iterator>
  #include <type_traits>
  @@ -49,3 +61,5 @@ public:
  constexpr reverse_iterator rend() const noexcept { return reverse_iterator{begin()}; }
  };
  } // namespace fizzy
  +
  +#endif /* __cplusplus > 201703L */
  