WIP: Allow tupleslots to have a fixed tupledesc, use in executor nodes.

WIP: Allow tupleslots to have a fixed tupledesc, use in executor nodes.

The reason for doing so is that it will allow expression evaluation to
optimize based on the underlying tupledesc. In particular it allows
JITing tuple deforming together with the expression itself.

For that expression initialization needs to be moved after the
relevant slots are initialized - mostly unproblematic, except in the
case of nodeWorktablescan.c.