Use OS-native directory separators in Path.FullPath

There are applications which do not understand the alternate directory separator; for example, https://github.com/cake-build/cake/issues/2337. There have also been multiple CLI tools I've used where I had to do .FullName.Replace('/', '\\').

Would you consider returning OS-native directory separators from Path.FullName? It will just work in all situations, and it will not stand out and look strange on Windows any longer even when it does work. (At times, I've manually edited MSBuild logs etc so people don't ask why I'm using weird paths.)