Relocate partition pruning structs to a saner place.

Relocate partition pruning structs to a saner place.

These struct definitions were originally dropped into primnodes.h,
which is a poor choice since that's mainly intended for primitive
expression node types; these are not in that category.  What they
are is auxiliary info in Plan trees, so move them to plannodes.h.

For consistency, also relocate some related code that was apparently
placed with the aid of a dartboard.

There's no interesting code changes in this commit, just reshuffling.

David Rowley and Tom Lane

Discussion: https://postgr.es/m/CAFj8pRBjrufA3ocDm8o4LPGNye9Y+pm1b9kCwode4X04CULG3g@mail.gmail.com