Write a function that transforms an array into an array of its differences repeatedly until there exists only one element left. A difference is A[n+1] - A[n].
To illustrate:
[5, 1, 9, 3, 4, 0]
[-4, 8, -6, 1, -4]
// 1 - 5 = -4; 9 - 1 = 8; 3 - 9 = -6; etc.
[12, -14, 7, -5]
[-26, 21, -12]
[47, -33]
-80
nDifferences([5, 1, 9, 3, 4, 0]) ➞ -80
nDifferences([1, 1, 1, 1]) ➞ 0
nDifferences([5, 8, 8]) ➞ -3
Each array will have at least two elements.