Remove tests passing negative arguments

This doesn't work because the Command library has some deficiencies...

One of them being that it's hard to stop processing arguments as flags
and then continue processing them. janestreet/core#24 has more info
about the way this currently works.
Swap GT and LT operators. They were switched
Make reduce nget's index by 1

Because list's are 0 indexed, but the postfix stack is 1 indexed.
Reimplement stack from scratch

Using do_int_op was accidentally enforcing that the values be integers