The same result could be achieved by introducing a smaller change to the language that doesn't completely redefine the way existing functions are evaluated. By using some specific signature (like adding a keyword to the first argument of a function), you could indicate that this function may be used as a member method of the first argument. For example, C# uses the keyword 'this' for its extension methods:

  public static class Helpers
  {
    public static string UrlEncode(this string url)
    {
      //...
    }
  }

  // then later:
  string someText = "...";
  string a = Helpers.UrlEncode(someText); // valid
  string b = someText.UrlEncode(); // valid too

But of course that would mean having to update all the function declarations of the standard library to add the new keyword when needed...