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.)

Start by locating the Path.FullPath/FullName implementation and any path-related tests. Check the current separator behavior on each supported OS, then confirm that FullName returns the native separator for the relevant platform without breaking existing path behavior.

Written by the indexing model from the issue text.