Allow int8_t optimized vector serialization

int8_t serialization is allowed, but not the optimized vector
serialization. Fix that.

src/span.h

@@ -287,9 +287,11 @@ Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
 // Helper functions to safely cast basic byte pointers to unsigned char pointers.
 inline unsigned char* UCharCast(char* c) { return reinterpret_cast<unsigned char*>(c); }
 inline unsigned char* UCharCast(unsigned char* c) { return c; }
+inline unsigned char* UCharCast(signed char* c) { return reinterpret_cast<unsigned char*>(c); }
 inline unsigned char* UCharCast(std::byte* c) { return reinterpret_cast<unsigned char*>(c); }
 inline const unsigned char* UCharCast(const char* c) { return reinterpret_cast<const unsigned char*>(c); }
 inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
+inline const unsigned char* UCharCast(const signed char* c) { return reinterpret_cast<const unsigned char*>(c); }
 inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }
 // Helper concept for the basic byte types.
 template <typename B>