Implements spot::ltl::destroy() and exercise it.

* src/ltlast/atomic_prop.hh: Declare instance_count().
* src/ltlast/binop.hh, src/ltlast/unop.hh, src/ltlast/multop.hh:
Likewise.  Also, really inherit for ref_formula this time.
* src/ltlast/atomic_prop.cc, src/ltlast/binop.cc,
src/ltlast/unop.cc, src/ltlast/multop.cc: On destruction, suppress
the instance from the instance map.  Implement instance_count().
* src/ltlast/formula.cc, src/ltlast/formula.hh,
src/ltlast/ref_formula.cc, src/ltlast/ref_formula.hh: Add virtual
destructors.
* src/ltlparse/ltlparse.yy: Recover from binary operators with
missing right hand operand (the point is just to destroy the
the left hand operand).
* src/ltltest/equals.cc, src/ltltest/readltl.cc,
src/ltltest/tostring.cc: Destroy used formulae.  Make sure
instance_count()s are null are the end.
* src/ltltest/parseerr.test: Adjust expected result, now
that the parser lnows about missing right hand operands.
* src/ltlvisit/destroy.hh, src/ltlvisit/destroy.cc,
src/ltlvisit/postfix.hh, src/ltlvisit/postfix.cc: New files.
* src/ltlvisit/Makefile.am (libltlvisit_la_SOURCES): Add them.
* src/ltlvisit/lunabbrev.cc (Xor, Equiv): Clone formulae
occurring twice in the rewritten expression.

src/ltlvisit/Makefile.am

@@ -5,6 +5,8 @@ noinst_LTLIBRARIES = libltlvisit.la
 libltlvisit_la_SOURCES = \
   clone.cc \
   clone.hh \
+  destroy.cc \
+  destroy.hh \
   dotty.cc \
   dotty.hh \
   dump.cc \
@@ -17,5 +19,7 @@ libltlvisit_la_SOURCES = \
   lunabbrev.cc \
   nenoform.hh \
   nenoform.cc \
+  postfix.hh \
+  postfix.cc \
   tunabbrev.hh \
-  tunabbrev.cc
+  tunabbrev.cc