Fixed ABI compatibility with libfmt.so.6.0.0 (#1471).

Fixed handling types convertible to std::string_view (#1451). Thanks @denizevrenci (Deniz Evrenci).

Made CUDA test an opt-in enabled via the FMT_CUDA_TEST CMake option.

Fixed sign conversion warnings (#1440). Thanks @0x8000-0000 (Florin Iucha).