With callback

Since memory for callbacks aren't automatically managed (anymore), I think a new word to make it easier would be good! It works like this:

    <make-callback> [ run code ] with-callback

And afterwards, all traces of the callback is gone. I'll make another commit after this one soon to fix the test cases which I've found to leak callback memory.


Since we have transposed assoc operations ``value-at``, we could make this ``delete-value-at`` and put it in the assocs vocabulary.  Although I might want to change it to stop on the first one found rather than filtering the whole thing.