Patch to inline stable SQL set returning UDFs

I discussed this hack with Neil and Josh in Ottawa back in May. I
took a stab at it a few weeks ago and below are the results. It is
mostly modeled after the existing inline_function. The approach taken
is to recursively cleanup the RTable where stable SQL functions are
found in it.

I've been testing this in our dev environment for a few weeks now.
I'm sure it still has issues but I think it is about as clean as I can
get it without help from a wider audience. This is my first backend
patch so please be gentle.

I want to thank Neil for answering many of my supremely ignorant
questions on IRC.

--
"An eye for eye only ends up making the whole world blind." -- Mohandas Gandhi