Call for objections: simplify stable functions during estimation

Awhile back, there was some discussion about pre-folding now() and
related functions when the planner is trying to estimate selectivities.
This would allow reasonable plans to be made for cases likeWHERE moddate >= current_date - 10;
without having to indulge in any crude hacks with mislabeled wrapper
functions, such as you can find all too often in the archives :-(

I was a bit hesitant about it at the time because I wasn't sure of all
the implications; but I've looked the idea over again, and as far as I
can see it's reasonable to pre-fold *all* stable functions when deriving
statistical estimates.  (One of the potential objections went away when
we started enforcing that stable functions don't have side-effects.)

The infrastructure for this is already there, because of Oliver Jowett's
previous work to teach eval_const_expressions() whether it's folding
the expression "for real" or just for estimation; it's basically a one
line change to treat stable functions differently in the two cases.

I know it's a bit late in the cycle, but I'd like to go ahead and make
this change for 8.0.  Objections?
        regards, tom lane