Simplify data structure for function names/variants

There is no need to store the number of characters in the signature.
Instead, use the marker value \cs{scan_stop:}, which typesets to
nothing.  This simplifies listing all the variants too.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5329 de43f980-851b-0410-b2f7-c40aca1f87e0