(BEGV_ADDR, PT_ADDR, ZV_ADDR): Use macro POS_ADDR.

(BEGV_ADDR, PT_ADDR, ZV_ADDR): Use macro POS_ADDR.
(Z_ADDR, BUF_GPT_ADDR, BUF_Z_ADDR): New macros.
(struct buffer): New members `category_table',
`direction_reserved', and `enable-multibyte-characters'.
(POS_ADDR): New macro used at all places instead of &FETCH_CHAR.
(FETCH_BYTE): New macro used at any places to check the argument
against an ASCII character.
(FETCH_MULTIBYTE_CHAR): New macro used at any places to get a
character code of the argument when it is known to be a multibyte
character.
(_fetch_multibyte_char_p, _fetch_multibyte_char_len): New
variables.
(FETCH_CHAR): Use macros FETCH_MULTIBYTE_CHAR and FETCH_BYTE.