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/varlena.c

@@ -5155,8 +5155,8 @@ text_format(PG_FUNCTION_ARGS)
 
 				str = OutputFunctionCall(&typoutputinfo_width, value);
 
-				/* pg_atoi will complain about bad data or overflow */
-				width = pg_atoi(str, sizeof(int), '\0');
+				/* pg_strtoint32 will complain about bad data or overflow */
+				width = pg_strtoint32(str);
 
 				pfree(str);
 			}