Optimizations for integer to decimal output.

Using a lookup table of digit pairs reduces the number of divisions
needed, and calculating the length upfront saves some work; these
ideas are taken from the code previously committed for floats.

David Fetter, reviewed by Kyotaro Horiguchi, Tels, and me.

Discussion: https://postgr.es/m/20190924052620.GP31596%40fetter.org

src/backend/access/common/printsimple.c

@@ -112,7 +112,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 			case INT8OID:
 				{
 					int64		num = DatumGetInt64(value);
-					char		str[23];	/* sign, 21 digits and '\0' */
+					char		str[MAXINT8LEN + 1];
 
 					pg_lltoa(num, str);
 					pq_sendcountedtext(&buf, str, strlen(str), false);