input: Write a python function to split a string into two equal parts.
  assert split_string("abcdefgh") == ["abcd", "efgh"]
  assert split_string("12345678") == ["1234", "5678"]
  assert split_string("hello!") == ["hel", "lo!"]
output: def split_string(s):
      mid = len(s) // 2
      return [s[:mid], s[mid:]]