(emacs_code_class_type): Delete the member

(emacs_code_class_type): Delete the member
EMACS_leading_code_composition.
(COMPOSING_NO) (COMPOSING_WITH_RULE_HEAD) (COMPOSING_NO_RULE_HEAD)
(COMPOSING_WITH_RULE_TAIL) (COMPOSING_NO_RULE_TAIL)
(COMPOSING_WITH_RULE_RULE) (COMPOSING_HEAD_P)
(COMPOSING_WITH_RULE_P): Macros deleted.
(COMPOSITION_DATA_SIZE) (COMPOSITION_DATA_MAX_BUNCH_LENGTH): New
macros.
(struct composition_data): New structure.
(CODING_FINISH_INSUFFICIENT_CMP): New macro.
(struct coding_system): New members composition_rule_follows,
cmp_data, cmp_data_start, cmp_data_index.
(coding_save_composition) (coding_free_composition_data)
(coding_adjust_composition_offset): Extern them.