The debugging features previously available via JQ_DEBUG are now command-line options.
After something is popped from a stack, we overwrite the memory with uninitialised data (if JQ_DEBUG is on). This means that valgrind reports use-after-pop as an uninitialised memory error.
Closes #43. Tested with: ulimit -v 5000 ./jq -n -c 'def f(x): x,f([x,x]); f(0)'
