Problem:
`IterArray:take()` iterates up to `self._tail`, but `_tail` is
exclusive. When no element fails the predicate, the loop reads one index
past the last element and calls the predicate with `nil`.
Solution:
Stop at `self._tail - inc`, which is the last in-range index for both
iteration directions. Add tests using a predicate which dereferences its
argument and matches every element, forward and reversed.
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