Re: WIP: Faster Expression Processing v4

I've been busily hacking away on this, trying to make things cleaner and
fix a couple of bugs I stumbled across. (Confusion between ExecQual and
ExecCheck, for instance - we apparently lack regression tests exercising
constraints-returning-null in corner cases such as table rewrite.) It
will probably be a day or two more before I'm done.

A couple of naming questions:

* I concur with Heikki's dislike for the file name execInterpExpr.c.
Would it make it any better to switch to execExprInterp.c? I think
that having all this stuff under a common pattern execExpr*.c would
be a good thing (and I notice you've already got one comment referring
to them that way ...)

* execQual.c doesn't seem to have a unifying reason to exist anymore.
It certainly has little to do with evaluating typical qual expressions;
what's left in there seems to be mostly concerned with SRFs. I feel
like it might be a good idea to rename it, but to what? execExprUtils.c
perhaps? Or maybe we should destroy it altogether, shoving the SRF
stuff into nodeFunctionscan.c and moving what little remains into
execUtils.c.

* I do not like the function name ExecInstantiateExpr(). Webster's
defines "instantiate" as "to represent (an abstraction) by a concrete
instance", which does not seem to me to have a lot to do with what this
function actually does. There's nothing very abstract about its input.
More, the implication of "instantiate" is that you can instantiate any
number of representatives of the same abstraction, but this scribbles
on the input in a one-way fashion. I think perhaps something like
ExecPrepareExpr or ExecFinishExpr or something along that line would
be better, but nothing is really standing out as le mot juste.

Thoughts?

regards, tom lane