pgsql: KNNGIST, otherwise known as order-by-operator support for GIST.

KNNGIST, otherwise known as order-by-operator support for GIST.

This commit represents a rather heavily editorialized version of
Teodor's builtin_knngist_itself-0.8.2 and builtin_knngist_proc-0.8.1
patches.  I redid the opclass API to add a separate Distance method
instead of turning the Consistent method into an illogical mess,
fixed some bit-rot in the rbtree interfaces, and generally worked over
the code style and comments.

There's still no non-code documentation to speak of, but I'll work on
that separately.  Some contrib-module changes are also yet to come
(right now, point <-> point is the only KNN-ified operator).

Teodor Sigaev and Tom Lane

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=554506871bd3a73a5d9fa4ee3aa0b0fbf406f8ab

Modified Files
--------------
src/backend/access/gist/gist.c             |   10 +
src/backend/access/gist/gistget.c          |  819 +++++++++++++++-------------
src/backend/access/gist/gistproc.c         |   97 ++++-
src/backend/access/gist/gistscan.c         |  179 +++++--
src/include/access/gist.h                  |   10 +-
src/include/access/gist_private.h          |  117 +++--
src/include/catalog/catversion.h           |    2 +-
src/include/catalog/pg_am.h                |    2 +-
src/include/catalog/pg_amop.h              |    1 +
src/include/catalog/pg_amproc.h            |    1 +
src/include/catalog/pg_proc.h              |    4 +-
src/include/utils/geo_decls.h              |    1 +
src/test/regress/expected/create_index.out |  136 +++++
src/test/regress/expected/opr_sanity.out   |   29 +-
src/test/regress/sql/create_index.sql      |   36 ++
src/test/regress/sql/opr_sanity.sql        |   26 +-
16 files changed, 972 insertions(+), 498 deletions(-)