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/arrayutils.c

@@ -226,8 +226,7 @@ ArrayGetIntegerTypmods(ArrayType *arr, int *n)
 	result = (int32 *) palloc(*n * sizeof(int32));
 
 	for (i = 0; i < *n; i++)
-		result[i] = pg_atoi(DatumGetCString(elem_values[i]),
-							sizeof(int32), '\0');
+		result[i] = pg_strtoint32(DatumGetCString(elem_values[i]));
 
 	pfree(elem_values);