Add a cardinality function for arrays.

Add a cardinality function for arrays.

Unlike our other array functions, this considers the total number of
elements across all dimensions, and returns 0 rather than NULL when the
array has no elements.  But it seems that both of those behaviors are
almost universally disliked, so hopefully that's OK.

Marko Tiikkaja, reviewed by Dean Rasheed and Pavel Stehule