Reorder definitions in parsenodes.h

This allows Stmt nodes to refer to smaller nodes. This is needed
for schemas as we plan to use RangeVar in several statements
(the option would be to define the field as struct RangeVar *,
but it is cleaner to use the type defined for that).

I am trying to do the reorg now so that any changes due to schemas
show up in a cleaner way.

I kept Query at the top for developers convenience.
Them it goes:
_ Supporting data structures for Parse Trees
_ Nodes for a Domain Creation tree (?)
_ Nodes for a Query tree
_ Optimizable Statements
_ Other Statements (no optimizations required)

I left the enums that are only used in one place close
to the definition that uses/is related to it.
The inhOption one is used in many places, so I moved
it closer to the includes at the top of the file.

The context diff results weird, so I've attached the resulting
file as well so people can look at it.

--
Fernando Nasser
Red Hat Canada Ltd. E-Mail: fnasser(at)redhat(dot)com
2323 Yonge Street, Suite #300
Toronto, Ontario M4P 2C9