(Fsubstitute_in_file_name): Adjusted for the change of

(Fsubstitute_in_file_name): Adjusted for the change of
CHAR_STRING.
(Finsert_file_contents): Set Vlast_coding_system_used before
calling signal_after_change.  Call update_compositions if some
texts are inserted..
(Fwrite_region): Adjusted for the change of a_write and e_write.
(a_write): Argument changed.  Work based on character position,
not byte position.
(e_write): Argument changed.  Handle new way of composition.