Added replace() function to strings.eel

API: function replace(str, what, with), where 'str' is
the source (any indexable type), 'what' is the character
or string to match in 'str, and 'with' isthe character
or string to substitute. Returns a new dstring.

NOTE: Only character (integer) supported for 'what' at
      this point! String matching to be implemented.