[PATCH] we have added support for box type in SP-GiST index

Hello, Hacker.

* [PATCH] add a box index to sp-gist

   We have extended sp-gist with an index that keeps track of boxes

   We have used ideas underlying sp-gist range implementation to
   represent 2D boxes as points in 4D space. We use quad tree
   analogue, but in 4-dimensional space. We call this tree q4d. Each
   node of this tree is a box (a point in 4D space) which splits space
   in 16 hyperrectangles.

   Rationale: r-tree assumes that boxes we're trying to index don't
   overlap much. When this assumption fails, r-tree performs badly,
   while our proposal to represent a rectangle as a point in 4D space
   solves this problem.

   NB: the index uses traversalValue introduced in a separate patch.

* [PATCH] add traversalValue in sp-gist

   During implementation of box index for sp-gist we saw that we only
   keep rectangles, but to determine traversal direction we may need
   to know boundaries of a hyperrectangle. So we calculate them
   on the fly and store them in traversalValue, available
   when traversing child nodes, because otherwise we would't be able to
   calculate them from inside the inner_consistent function call.

   This patch was written by Teodor Sigaev.

* [PATCH] change reconstructValue -> traversalValue in range_spgist.c

   During traversal, local context is
   insufficient to pick traversal direction. As of now, this is worked
   around with the help of reconstructValue. reconstructValue only
   stores data of the same type as a tree node, that is, a range.

   We have written a patch that calculates auxillary values and makes
   them accessible during traversal.

   We then use traversalValue in a new box index and change
   range_spgist.c to use it in place of reconstructValue.

   NB: apply this patch after traversalValue patch.