(move_gap): Use move_gap_both.

(move_gap): Use move_gap_both.
(move_gap_both): New function.
(gap_left, gap_right): Take both charpos and bytepos args.
(adjust_markers_gap_motion): Renamed from adjust_markers and simplified.
(adjust_markers_for_delete): New function.
(adjust_markers_for_insert): Take args in chars and bytes.
Also new arg BEFORE_MARKERS.  One call does all marker updating
needed for any insert.
(adjust_point): Take 2 args and update PT and PT_BYTE.
(make_gap): Handle bytes vs chars.
(insert, insert_and_inherit): Handle bytes vs chars.
Pass new BEFORE_MARKERS arg to insert_1.
(insert_before_markers, insert_before_markers_and_inherit): Likewise.
(insert_from_string, insert_from_string_before_markers): Likewise.
(insert_from_buffer): Likewise.
(insert_1): Handle bytes vs chars.  New arg BEFORE_MARKERS.
(insert_from_string_1, insert_from_buffer_1): Likewise.
(replace_range): Likewise.
(del_range_2): New subroutine, taken from del_range_1.
(del_range_1): Use del_range_2.
(del_range_byte, del_range_both): New functions.