unabbreviate: enable removal of R
This implies learning alternative rules for G, and W as well, since those would use R. Fixes #103. Suggested by Joachim Klein. * src/ltlvisit/unabbrev.cc, src/ltlvisit/unabbrev.hh: Implement the new rules. * doc/tl/tl.tex: Document the rules. * src/tests/unabbrevwm.test: Test them. * src/bin/ltlfilt.cc, NEWS: Mention that --unabbreviate accepts R.