input: Write a python function to square each number in a list.
  assert to_square([1, 2, 3, 4, 5]) == [1, 4, 9, 16, 25]
  assert to_square([0, -1, -2, -3, -4]) == [0, 1, 4, 9, 16]
  assert to_square([10, 20, 30]) == [100, 400, 900]
output: def to_square(lst):
      return [x**2 for x in lst]