Getting rid of legacy cruft

[rkaminsk]

I implemented support for constraints referring to the future, which involves a lot of ugly code and complicated processing. This can be simplified by mapping to other language constructs. For example, the rule

:- p(X), q'(X,Y), not r''(Y).

can be rewritten to

:- 'p(X), q(X,Y), not &tel { > r(Y) }, <ground-later-magic>.

The <ground-later-magic> just has to ensure that the rule is grounded after the amount of time steps it has been shifted.

Note that we cannot replace q'(X,Y) by a &tel formula because those cannot bind variables.