Save the old keyboard-translate-table

In case the user had any other settings in keyboard-translate-table
before using keyswap-mode, it now saves the table in the variable
keyswap-original-table before doing the swaps, then resets
keyboard-translate-table back to it when the mode is disabled. This is
better, but any additional changes made to the keyboard-translate-table
_while_ keyswap-mode was enabled will still be lost when it is disabled.