It is limited to negative_normal_form_visitor for now.

* src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh: New files.
* src/ltlvisit/Makefile.am: Add them.
* src/ltlvisit/nenoform.cc, src/ltlvisit/nenoform.hh: Rewrite
using ltl_simplifier.