BETWEEN SYMMETRIC

Hi,

Based on recent discussion, I went thru and got together the work I'd done
on the BETWEEN node.  It's not as far along as I thought.  I ran into a few
hurdles:

* ExecEvalBetweenExpr is probably beyond my powers - I've done my best and
marked my hopelessness with '@@' symbols.  I don't know how to actually
evaluate the node properly, I don't know how to check that all the 3 types
are coercible to the same type and I don't know how to make it take rowvars
(sic?)instead of scalars, as per spec.

Copy and Equal are done, I think.

Out I've guessed at how to do it based on other examples, but I need
feedback.  Read I haven't done at all cos I don't quite understand when/why
it's used or how to do it.

The grammar has been updated to use the new BetweenExpr node, with new
syntax options.

The new keywords have been added in the relevant places, and they are
reserved.

nodes.h and parsenodes.h are aware of the new node.

I have added a full regression test that I used in my old gram.y only
implementation, that didn't use a new node - it will be helpful!

Where do we go from here?

Chris