Suggested by Joachim Klein.
When a bogus formula is found by ltlcross, the --grind=FILENAME option
tries to find a smaller formula for which the bug is still present, and
outputs it in FILENAME.
* src/bin/ltlcross.cc: Add the --grind option.
* doc/org/ltlcross.org: Document the --grind option.
* src/ltltest/ltlcrossgrind.test: Test it.
* src/ltltest/Makefile.am: Add test.