22a. Sum Digits

sum-digits: whole-number -> number

Find the sum of the digits in a number.

n	(sum-digits n)	simplified
5	5	5
53	5 + 3	8
921	9 + 2 + 1	12
3216	3 + 2 + 1 + 6	12
65536	6 + 5 + 5 + 3 + 6	25