Updated \<thing>_case:nn(TF) code

This uses Bruno's approach to allow T/F branches without too much overhead
and without altering the test used for ending recursion. Documentation has
been adjusted to match.

Test cases are probably needed for all of the branches: will do that a little
later!


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4565 de43f980-851b-0410-b2f7-c40aca1f87e0