Hand code string to integer conversion for performance.

As benchmarks show, using libc's string-to-integer conversion is
pretty slow. At least part of the reason for that is that strtol[l]
have to be more generic than what largely is required inside pg.

This patch considerably speeds up int2/int4 input (int8 already was
already using hand-rolled code).

Most of the existing pg_atoi callers have been converted. But as one
requires pg_atoi's custom delimiter functionality, and as it seems
likely that there's external pg_atoi users, it seems sensible to just
keep pg_atoi around.

Author: Andres Freund
Reviewed-By: Robert Haas
Discussion: https://postgr.es/m/20171208214437.qgn6zdltyq5hmjpk@alap3.anarazel.de

src/backend/utils/adt/int8.c

@@ -101,6 +101,7 @@ scanint8(const char *str, bool errorOK, int64 *result)
 
 	if (!neg)
 	{
+		/* could fail if input is most negative number */
 		if (unlikely(tmp == PG_INT64_MIN))
 			goto out_of_range;
 		tmp = -tmp;