input: Write a function to create a dictionary where each key is an element from the given list and the corresponding value is the square of that key. 
  assert square_dict([1, 2, 3, 4, 5]) == {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  assert square_dict([]) == {}
  assert square_dict([0, -1, -2, 3]) == {0: 0, -1: 1, -2: 4, 3: 9}
output: def square_dict(lst):
      return {x: x**2 for x in lst}