Make expandable sort use \exp_not:n. Rename auxiliaries.

Just like \tl_head:n, \seq_item:Nn and the like, \tl_sort:nN
now returns its result wrapped within \exp_not:n.  I also
found an important optimization to make, see next commit
(as it is now, a couple thousand items exhaust TeX's
3000000 main memory).


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4488 de43f980-851b-0410-b2f7-c40aca1f87e0