Add the QUIT word.

- it ends the current evaluation immediately, leaving the machine in
whatever state it had been in before;
- this includes not clearing the stack or resetting the mode, as errors
do. QUIT is an expected and perfectly okay thing to do.
Comparisons, pairwise stack handling, and LEAVE.

In writing the test for LEAVE, it turned out I didn't have comparison
operators, so I wrote the test for comparison operators, but needed 2DUP
to make the test easier, so I wrote the test for 2DUP/2SWAP/2OVER.

Then I popped each one off the stack and wrote the code for it.