input: Write a function to take a list and return a dictionary with the elements as keys and the product of each element as values.
  assert product_of_list([7, 8, 5])=={7: 49, 8: 64, 5: 25}
  assert product_of_list([3, 4, 6])=={3: 9, 4: 16, 6: 36}
  assert product_of_list([2, 1, 9, 12])=={2: 4, 1: 1, 9: 81, 12: 144}
  assert product_of_list([10, 5, 0])=={10: 100, 5: 25, 0: 0}
  assert product_of_list([])=={}
output: def product_of_list(lst):
      return {x: x * x for x in lst}