Have the planner convert COUNT(1) / COUNT(not_null_col) to COUNT(*)

Since e2debb643, we've had the ability to determine if a column is
NULLable as early as during constant folding. This seems like a good
time to consider converting COUNT(not_null_col) into COUNT(*), which
is faster and may result in far fewer columns being deformed from the
tuple.

To make this work, I invented "SupportRequestSimplifyAggref", which is
similar to the existing SupportRequestSimplify, which is for
FuncExprs. Aggregates use Aggrefs, so we need something else.

It's easy to see that count(*) is faster. Here's a quick test in an
unpatched master:

create table t (a int, b int, c int, d int, e int, f int, g int, h int
not null);
insert into t (h) select 1 from generate_Series(1,1000000);
vacuum freeze t;

master:
select count(h) from t;
Time: 16.442 ms
Time: 16.255 ms
Time: 16.322 ms

master:
select count(*) from t;
Time: 12.203 ms
Time: 11.402 ms
Time: 12.054 ms (+37%)

With the patch applied, both queries will perform the same.

It may be possible to apply transformations to other aggregate
functions too, but I don't want to discuss that here. I mostly want to
determine if the infrastructure is ok and do the count(*) one because
it seems like the most likely one to be useful.

One thing I wasn't too sure about was if we should make it possible
for the support function to return something that's not an Aggref. In
theory, something like COUNT(NULL) could just return '0'::bigint.
While that does seem an optimisation that wouldn't be applied very
often, I have opted to leave it so that such an optimisation *could*
be done by the support function. I also happen to test that that
doesn't entirely break the query, as ordinarily it would if we didn't
have Query.hasAggs (It's not too dissimilar to removing unused columns
from a subquery)

Should we do this?

David