Changes

• Pull request #112, lots of cleanup work on the readme, thanks!
• Enhancement for #111, further optimization of directory iteration, performance for recursive_directory_iterator over large trees now somewhere between libc++ and libstdc++.
• Enhancement for #110, ghc::filesystem now has preliminary support for Cygwin. Changes where made to allow the tests to compile and run successfully (tested with GCC
  10.2.0), feedback and additional PRs welcome as it is currently not part of the CI configuration.
• Pull request #109, various spelling errors in error messages and comments fixed.
• Pull request #108, old style casts removed.
• Fix for #107, the error handling for status calls was suppressing errors on symlink targets.
• Pull request #106, fixed detection of AppleClang for compile options.
• Pull request #105, added option GHC_FILESYSTEM_BUILD_STD_TESTING to override additional build of std::filesystem versions of the tests for comparison and the possibility to use GHC_FILESYSTEM_TEST_COMPILE_FEATURES to prefill the used compile features defaulting to CMAKE_CXX_COMPILE_FEATURES when not given.