input: Write a function to find the smallest multiple of all numbers in a list.
  assert smallest_multiple([1, 2, 3, 4, 5, 6]) == 60
  assert smallest_multiple([2, 3, 4, 5]) == 60
  assert smallest_multiple([6, 8, 12]) == 24
output: from math import gcd
  from functools import reduce
  
  def smallest_multiple(numbers):
      def lcm(a, b):
          return a * b // gcd(a, b)
      return reduce(lcm, numbers)