message: Add MakeUCharSpan, to help constructing Span<[const] unsigned char>
  Based on a suggestion by Russell Yanofsky.
diff: @@ -207,4 +207,16 @@ T& SpanPopBack(Span<T>& span)
  return back;
  }
  +// Helper functions to safely cast to unsigned char pointers.
  +inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; }
  +inline unsigned char* UCharCast(unsigned char* c) { return c; }
  +inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; }
  +inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
  +
  +// Helper function to safely convert a Span to a Span<[const] unsigned char>.
  +template <typename T> constexpr auto UCharSpanCast(Span<T> s) -> Span<typename std::remove_pointer<decltype(UCharCast(s.data()))>::type> { return {UCharCast(s.data()), s.size()}; }
  +
  +/** Like MakeSpan, but for (const) unsigned char member types only. Only works for (un)signed char containers. */
  +template <typename V> constexpr auto MakeUCharSpan(V&& v) -> decltype(UCharSpanCast(MakeSpan(std::forward<V>(v)))) { return UCharSpanCast(MakeSpan(std::forward<V>(v))); }
  +
  #endif
  