input: Write a function to create a dictionary that contains the structure [x:x^2] where x is the key and x^2 is the value, given a list of numbers as input. If the input list is empty, return an empty dictionary.
  assert square_dictionary([1, 2, 3, 4, 5])=={1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  assert square_dictionary([0, -1, -2])=={0: 0, -1: 1, -2: 4}
  assert square_dictionary([])=={}
output: def square_dictionary(numbers):
      return {x: x**2 for x in numbers}