sum() on int2 and int4 columns now uses an int8, not numeric, accumulator

for speed reasons; its result type also changes to int8.  avg() on these
datatypes now accumulates the running sum in int8 for speed; but we still
deliver the final result as numeric, so that fractional accuracy is
preserved.

count() now counts and returns in int8, not int4.  I am a little nervous
about this possibly breaking users' code, but there didn't seem to be
a strong sentiment for avoiding the problem.  If we get complaints during
beta, we can change count back to int4 and add a "count8" aggregate.
For that matter, users can do it for themselves with a simple CREATE
AGGREGATE command; the int4inc function is still present, so no C hacking
is needed.

Also added max() and min() aggregates for OID that do proper unsigned
comparison, instead of piggybacking on int4 aggregates.

initdb forced.

src/test/regress/expected/opr_sanity.out

@@ -480,8 +480,8 @@ WHERE p1.aggtransfn = p2.oid AND
           (p2.pronargs = 1 AND p1.aggbasetype = 0)));
   oid  | aggname | oid |   proname   
 -------+---------+-----+-------------
- 10020 | max     | 768 | int4larger
- 10034 | min     | 769 | int4smaller
+ 10021 | max     | 768 | int4larger
+ 10036 | min     | 769 | int4smaller
 (2 rows)
 
 -- Cross-check finalfn (if present) against its entry in pg_proc.

src/test/regress/expected/rules.out

@@ -753,7 +753,7 @@ create view rtest_vview4 as select X.a, X.b, count(Y.a) as refcount
 	where X.a = Y.a
 	group by X.a, X.b;
 create function rtest_viewfunc1(int4) returns int4 as
-	'select count(*) from rtest_view2 where a = $1'
+	'select count(*)::int4 from rtest_view2 where a = $1'
 	language 'sql';
 create view rtest_vview5 as select a, b, rtest_viewfunc1(a) as refcount
 	from rtest_view1;

src/test/regress/sql/rules.sql

@@ -422,7 +422,7 @@ create view rtest_vview4 as select X.a, X.b, count(Y.a) as refcount
 	where X.a = Y.a
 	group by X.a, X.b;
 create function rtest_viewfunc1(int4) returns int4 as
-	'select count(*) from rtest_view2 where a = $1'
+	'select count(*)::int4 from rtest_view2 where a = $1'
 	language 'sql';
 create view rtest_vview5 as select a, b, rtest_viewfunc1(a) as refcount
 	from rtest_view1;