Speed up replace-buffer-contents

Speed up replace-buffer-contents

* src/editfns.c (EXTRA_CONTEXT_FIELDS): Add a_unibyte and
b_unibyte members.
(rbc_quitcounter): New static variable.
(Freplace_buffer_contents): Initialize a_unibyte, b_unibyte, and
rbc_quitcounter.  Inhibit modification hooks if they were not
already inhibited.  Use rarely_quit to allow user to quit, to
avoid calling maybe_quit too frequently (which hurts performance).
Remove redundant assertions (which hurt performance too much).
Call signal_after_change and update_compositions after all the
changes are done.
(buffer_chars_equal): Remove redundant assertions (which hurt
performance).  Avoid using BUF_FETCH_CHAR_AS_MULTIBYTE, which
hurts performance by referencing Lisp symbols; instead, use
lower-level macros with explicit tests to select which macro to
use.  (Bug#31888)