[boost] multiprecision issues

Hi John, i stumbled over some more issues https://godbolt.org/z/x5srG9eof : - the unary operator- does not work with boost::multiprecision::cpp_bin_float_N on nan - operator<<(boost::multiprecision::float128) does not support std::showpos with std::hexfloat - operator<<(boost::multiprecision::cpp_bin_float_N) does not support std::hexfloat and in the case of nan does not support std::showpos I also miss C++17 from_chars/to_chars. I realize that this is extremely tedious work, but multiprecision should be as compatible to the standard as possible. If you want, I'll be happy to look for various issues and report them to you, but you'll have to fix them yourself. cu Gero