input: Write a function called `toggle_case` that takes a single string as input and returns a new string with the case of each character reversed (i.e., uppercase letters become lowercase and vice versa). If the input string is empty, the function should return an empty string.
  
  assert toggle_case("Hello World")=="hELLO wORLD"
  assert toggle_case("Python")=="pYTHON"
  assert toggle_case("123ABC")=="123abc"
  assert toggle_case("aBcD")=="AbCd"
  assert toggle_case("")==""
output: def toggle_case(s):
      return s.swapcase()