Add this test for stacked 'apply'

[masak]

Rarely used in practice, it is nevertheless completely legal and well-defined to apply an apply, as if it were a function and not a hybrid between a special form and an evaluator special case:

$ bel
Language::Bel 0.63 -- msys.
> (set L '(1 2 3 4))
(1 2 3 4)
> (list L)
((1 2 3 4))
> (apply list L)
(1 2 3 4)
> (apply apply list L)
(1 2 3 . 4)
> (apply apply apply list L)
(1 2 3 lit num (+ (t t t t) (t)) + nil (t))
> (apply apply apply apply list L)
(1 2 3 lit num (+ (t t t t) (t)) + nil t)
> (apply apply apply apply apply list L)
*hangs*

I can kind of explain all the results to myself, except for the final hang. A guess is that we're expecting a nil at the end and never get it. Need to confirm this with a strict reading of the bel.bel evaluator source code.