(fresh (x y z) g1 g2 ...) expands to a let that calls (make-var) for each
named var, then mk-conjs the goals. call-fresh is the function-shaped
alternative for programmatic goal building.
9 new tests: empty-vars, single var, multi-var multi-goal, fresh under
disj, nested fresh, call-fresh equivalents. 91/91 cumulative.
