For a given list [x1, x2, x3, ..., xn] compute the last (decimal) digit of
x1 ^ (x2 ^ (x3 ^ (... ^ xn))).
last_digit([3, 4, 2]) == 1
because 3 ^ (4 ^ 2) = 3 ^ 16 = 43046721.
Beware: powers grow incredibly fast. For example, 9 ^ (9 ^ 9) has more
than 369 millions of digits. last_digit has to deal with such numbers
efficiently.
Corner cases: we assume that 0 ^ 0 = 1 and that last_digit of an empty
list equals to 1.