Extend cube on-disk format to pack points more tightly.

If the lower left and upper right corners of a cube are the same, set a
flag in the cube header, and only store one copy of the coordinates. That
cuts the on-disk size into half for the common case that the cube datatype
is used to represent points rather than boxes.

The new format is backwards-compatible with the old one, so pg_upgrade
still works. However, to get the space savings, the data needs to be
rewritten. A simple VACUUM FULL or REINDEX is not enough, as the old
Datums will just be moved to the new heap/index as is. A pg_dump and
reload, or something similar like casting to text and back, will do the
trick.

This patch deliberately doesn't update all the alternative expected output
files, as I don't have access to machines that produce those outputs. I'm
not sure if they are still relevant, but if they are, the buildfarm will
tell us and produce the diff required to fix it. If none of the buildfarm
animals need them, they should be removed altogether.

Patch by Stas Kelvich.