Clean up l3str.dtx, rename str_substr to str_range

The behaviour of str_range has also changed: it does not
accept empty arguments anymore.  For compatibility with
older code, str_substr remains for a bit, and detects
empty arguments.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4450 de43f980-851b-0410-b2f7-c40aca1f87e0