cache type info in json_agg and friends

Currently, json_agg, jsonb_agg, json_object_agg and jsonb_object_agg do
type classification on their arguments on each call to the transition
function. This is quite unnecessary, as the argument types won't change.
This patch remedies the defect by caching the necessary values in the
aggregate state object.

While this doesn't change the performance much, since these functions
are essentially dominated by other bits of the processing, I think it is
nevertheless worth doing.

There are other areas where we might attack this, also. In particular,
if one of the arguments is a record, then composite_to_json(b) will do
this for every attribute of every record. However, it's much less clear
to me how we can cache this information sensibly.

cheers

andrew