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/ltltest/parseerr.test

@@ -33,17 +33,19 @@ check()
 # Empty or unparsable strings
 check '' ''
 check '+' ''
-check 'a U' ''
-check 'a U b V c R' ''
 
 # leading and trailing garbage are skipped
 check '/2/3/4/5 a + b /6/7/8/' 'multop(Or, AP(a), AP(b))'
 check 'a U b c' 'binop(U, AP(a), AP(b))'
-check 'a &&& b' 'AP(b)'
+check 'a &&& b' 'multop(And, constant(0), AP(b))'
 # (check multop merging while we are at it)
 check 'a & b & c & d e' 'multop(And, AP(a), AP(b), AP(c), AP(d))'
 check 'a & (b | c) & d should work' 'multop(And, AP(a), multop(Or, AP(b), AP(c)), AP(d))'
 
+# Binop recovery
+check 'a U' 'constant(0)'
+check 'a U b V c R' 'constant(0)'
+
 # Recovery inside parentheses
 check 'a U (b c) U e R (f g <=> h)' \
       'binop(R, binop(U, binop(U, AP(a), AP(b)), AP(e)), AP(f))'