ghc::filesystem Release v1.4.0

Changes:

• Enhancements for #71, when compiled with C++20:
  • char8_t and std::u8string are supported where Source is the parameter type
  • fs::path::u8string() and fs::path::generic_u8string() now return a std::u8string
  • The spaceship operator <=> is now supported for fs::path
  • With the define GHC_FILESYSTEM_ENFORCE_CPP17_API ghc::filesystem will fall back to the old fs::path::u8string() and fs::path::generic_u8string() API if preferred
• Bugfix for fs::proximate(p, ec) where the internal call to fs::current_path() was not using the error_code variant, throwing possible exceptions instead of setting ec.
• Enhancement LWG_2936_BEHAVIOUR is now on by default.
• Some cleanup work to reduce preprocessor directives for better readability and remove unneeded template specializations.