Write a function square_nums that takes an array of integers as an argument and
returns a new array with the squares of all of the values of the passed array.
For example, if an array named $arr stores the values [0, 1, 2, 3, 4, 5],
the call square_nums($arr) should return a new array containing the values
[0, 1, 4, 9, 16, 25]. If $arr had instead contained [-1, 4, -5, 25],
the call square_nums($arr) would be expected to return a new array with the values
[1, 16, 25, 625].
Constraints:
- Do not modify the passed array.
