Explicitly invoke atom garbage collector per iter

It seems like we get unconstrained growth in the number of
atoms generated (maybe we never hit some kind of ceiling)
from the calls to jpl_new:

-------8<----------
main :- do_loop(10).

do_loop(N) :- N =< 0.
do_loop(N) :-
        jpl_new('java.lang.Integer', [0], _),
        %garbage_collect_atoms,
        statistics(atoms, NumAtoms),
        write(NumAtoms), nl,
        Nm1 is N-1,
        do_loop(Nm1).
-------8<----------