Toplevel should use copy_term/3 to show residual goals

[triska]

Here are several interrelated issues resulting from the desire to correctly show residual goals on the toplevel.

Starting with clpz10.pl from https://www.metalevel.at/clpz/scryer/clpz10.pl and the following program:

:- use_module(clpz10).

puzzle([S,E,N,D] + [M,O,R,E] = [M,O,N,E,Y]) :-
        Vars = [S,E,N,D,M,O,R,Y],
        Vars ins 0..9,
        all_different(Vars),
                  S*1000 + E*100 + N*10 + D +
                  M*1000 + O*100 + R*10 + E #=
        M*10000 + O*1000 + N*100 + E*10 + Y,
        M #\= 0, S #\= 0.

I get:

?- puzzle(Ps).
Ps = ([9,_751,_1102,_1453]+[1,0,_2506,_751]=[1,0,_1102,_751,_2857]) ;
false.

However, these variables are involved in constraints, and I therefore expect to see residual constraints.

I can obtain them as a list Gs of goals with copy_term/3:

?- puzzle(Ps), copy_term(Ps, Copy, Gs).

In this example, Copy is a fresh copy of the term Ps, and I therefore expect the following query to succeed, although it currently unexpectedly fails:

?- puzzle(Ps), copy_term(Ps, Copy, Gs), Ps = Copy.
false.

In particular, the following should succeed, and the toplevel should use this to obtain the list Gs of residual goals:

?- puzzle(Ps), copy_term(Ps, Ps, Gs).

Currently, it unexpectedly fails, and the toplevel does not show the residual goals at all.

After this query has failed, the earlier query suddenly yields much more output, and the output also contains terms that are not residual goals, such as:

?- puzzle(Ps), copy_term(Ps, Copy, Gs).
..., clpz, u33, ...

Please see #253 for a crash that is also related to this.

[triska]

This now works correctly, thank you a lot!