* src/character.h (str_to_multibyte):
* src/character.c (str_to_multibyte): Remove `nbytes` argument; return
it instead.  Copy forwards.
* src/fns.c (concat_to_string, Fstring_make_multibyte):
Use str_to_multibyte.
(string_make_multibyte): Remove.
(string_to_multibyte):
* src/print.c (print_string):  Adapt calls.

* src/character.h (str_to_multibyte):
* src/character.c (str_to_multibyte): Change signature and simplify;
the conversion is no longer done in-place.
* src/fns.c (string_to_multibyte): Drop temporary buffer and memcpy;
adapt to new str_to_multibyte signature.
* src/print.c (print_string): Drop memcpy; adapt call to str_to_multibyte.
* test/src/fns-tests.el (fns--string-to-unibyte): Rename to...
(fns--string-to-unibyte-multibyte): ... this and strengthen, so that
the test covers string-to-multibyte reasonably well.

* src/character.h (str_to_unibyte):
* src/character.c (str_to_unibyte): Remove.
* src/fns.c (Fstring_to_unibyte): Ditch the call to str_to_unibyte and
the unnecessary heap allocation.  Write new, faster code.
* test/src/fns-tests.el (fns--string-to-unibyte): New test.

This function is used in many places to calculate the length of
a unibyte string converted to multibyte.

* src/character.c (count_size_as_multibyte): Move the overflow test
outside the loop, which makes it much faster.  Standard compilers
will even vectorise it if asked to (-O2 in Clang, -O3 in GCC).

* src/composite.c (find_automatic_composition): Accept one
additional argument BACKLIM; don't look back in buffer or string
farther than that.  Add an assertion for BACKLIM.
(composition_adjust_point, Ffind_composition_internal): Callers
adjusted.
* src/composite.h (find_automatic_composition): Adjust prototype.
* src/character.c (lisp_string_width): Call
'find_automatic_composition' with the value of BACKLIM equal to POS,
to avoid costly and unnecessary search back in the string, since
those previous characters were already checked for automatic
compositions.  (Bug#48734)  (Bug#48839)

'lisp_string_width' is called from 'format' and 'format-message',
which can be called both very early into Emacs initialization and in
other contexts where using the font backend is impossible or
undesirable.  So this commit changes 'lisp_string_width' to try
accounting for automatic compositions only when explicitly requested,
and only 'string-width' does that; 'format' and 'format-message'
don't.
* src/character.c (lisp_string_width): Accept an additional
argument AUTO_COMP; attempt accounting for auto-compositions only
if that argument is non-zero.  (Bug#48732)
* src/editfns.c (styled_format):
* src/character.c (Fstring_width): Callers of 'lisp_string_width'
adjusted.

* src/character.c (lisp_string_width): Compute C pointer to data
of STRING immediately before using it, since STRING could be
relocated by GC triggered by processing compositions.  (Bug#48711)

* src/character.c (lisp_string_width): Validate the value of TO
argument before passing it to 'composition_gstring_width'.
(Bug#48711)