code: public Signature argName(int index, String name) {
          String[] argNames = Arrays.copyOf(argNames(), argNames().length);
          argNames[index] = name;
          return new Signature(type(), argNames);
      }
docs: Set the argument name at the given index.
  
  @param index the index at which to set the argument name
  @param name the name to set
  @return a new signature with the given name at the given index