Improve doprnt performance

Improve doprnt performance

This patch implements some of my suggestions in Bug#8545,
with further changes suggested by Eli Zaretskii (Bug#43439).
* src/doprnt.c: Improve comments.
(SIZE_BOUND_EXTRA): Now at top level, for parse_format_integer.
(parse_format_integer): New static function, containing some of
the old doprnt.  Fix a bug that caused doprnt to infloop on
formats like "%10s" that Emacs does not use.  We could simplify
doprnt further if we dropped support for these never-used formats.
(doprnt_nul): New function.
(doprnt): Use it.  Change doprnt API to exit when either it finds NUL
or reaches the character specified by FORMAT_END.  In the typical case
where FORMAT_END is null, take just one pass over FORMAT, not two.
Assume C99 to make code clearer.  Do not use malloc or alloca to
allocate a copy of the format FMTCPY; instead, use a small fixed-size
array FMTSTAR, and use '*' in that array to represent width and
precision, passing them as separate int arguments.  Use eassume to
pacify GCC in switch statements.