Re: UNNEST with multiple args, and TABLE with multiple funcs

Here is a new patch with the following changes on top of Heikki's
version (all the changes in which I've otherwise kept):

1. Changed TupleDescCopyEntry parameter order to be (dest,src) for
   better consistency with TupleDescInitEntry and general C style.

2. Removed CreateTupleDescCopyExtend which is now dead code

3. Some small cleanups in the building of tupdescs in executor init

4. Refactored the grammar further to reinstant multi-arg unnest inside
   table(), and added regression tests for that

5. comment and doc changes requested in Heikki's message

--
Andrew (irc:RhodiumToad)