input: Write a python function to check if a list contains only prime numbers.
  assert are_all_primes([11, 13]) == True
  assert are_all_primes([11, 15]) == False
  assert are_all_primes([2, 3, 5, 7]) == True
  assert are_all_primes([2, 4, 6, 8]) == False
output: def are_all_primes(lst):
      def is_prime(n):
          if n <= 1:
              return False
          if n <= 3:
              return True
          if n % 2 == 0 or n % 3 == 0:
              return False
          i = 5
          while i * i <= n:
              if n % i == 0 or n % (i + 2) == 0:
                  return False
              i += 6
          return True
      
      return all(is_prime(num) for num in lst)