Write a function that returns the number of times you must carry a digit when you sum together two integers.
carry_digits(36, 135) ➞ 1
# You carry the 1 when you sum 6 and 5 together.
carry_digits(671, 329) ➞ 3
carry_digits(1111, 3333) ➞ 0
carry_digits(53214, 56905) ➞ 3
Count all carry operations (even those on leading digits).