g_unichar must be Crystal Char, not UInt32

[hugopl]

Gtk::TextIter have some methods that returns gunichar, the generator is translating them to UInt32 instead of Char.

    def char : UInt32
      # gtk_text_iter_get_char: (Method)
      # Returns: (transfer none)

      # C call
      _retval = LibGtk.gtk_text_iter_get_char(to_unsafe)

      # Return value handling
      _retval
    end