Rename \<thing>_case:nnn to \<thing>_case:nnF

This is step one of the process: just changing the code. The TF/T versions will
need a different implementation (I have one but need to add it carefully). Once
the TF versions are added, another update will be made to the docs.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4559 de43f980-851b-0410-b2f7-c40aca1f87e0