input: Write a python function to find the highest common factor (HCF) of two numbers.
  assert highest_common_factor(8, 12) == 4
  assert highest_common_factor(14, 28) == 14
  assert highest_common_factor(100, 200) == 100
  assert highest_common_factor(3, 7) == 1
output: def highest_common_factor(a, b):
      while b:
          a, b = b, a % b
      return a