Avoid calls to CHAR_TO_BYTE if byte position is known.

Avoid calls to CHAR_TO_BYTE if byte position is known.
* editfns.c (make_buffer_string_both): Use move_gap_both.
(Fbuffer_string): Use make_buffer_string_both.
* marker.c (buf_charpos_to_bytepos): Convert to eassert.
Adjust comment.
(buf_bytepos_to_charpos): Likewise.
(charpos_to_bytepos): Remove.
* fileio.c (Finsert_file_contents): Use move_gap_both.
* search.c (Freplace_match): Likewise.
* process.c (process_send_region): Likewise. Use convenient
names for byte positions.
* lisp.h (charpos_to_bytepos): Remove prototype.
* indent.c (scan_for_column): Use CHAR_TO_BYTE.
* insdel.c (move_gap): Likewise.