(PUP-840) FFI Puppet::Util::Colors

 - Replace WriteConsole / WriteConsoleOutputCharacter calls with FFI
   equivalents
 - Remove string_encode function as it's no longer necessary and the
   test that was using it
Merge pull request #2676 from Iristyle/ticket/master/PUP-840-FFI-Windows-Colors

(PUP-840) FFI Puppet::Util::Colors