pgsql: Add optional compression method to SP-GiST

Add optional compression method to SP-GiST

Patch allows to have different types of column and value stored in leaf tuples
of SP-GiST. The main application of feature is to transform complex column type
to simple indexed type or for truncating too long value, transformation could
be lossy.  Simple example: polygons are converted to their bounding boxes,
this opclass follows.

Authors: me, Heikki Linnakangas, Alexander Korotkov, Nikita Glukhov
Reviewed-By: all authors + Darafei Praliaskouski
Discussions:
https://www.postgresql.org/message-id/5447B3FF.2080406@sigaev.ru
https://www.postgresql.org/message-id/flat/54907069.1030506@sigaev.ru#54907069.1030506@sigaev.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/854823fa334cb826eed50da751801d0693b10173

Modified Files
--------------
doc/src/sgml/spgist.sgml                | 92 ++++++++++++++++++++++++++-------
src/backend/access/spgist/spgdoinsert.c | 37 ++++++++++---
src/backend/access/spgist/spgscan.c     |  6 +--
src/backend/access/spgist/spgutils.c    | 21 +++++++-
src/backend/access/spgist/spgvalidate.c | 50 +++++++++++++++++-
src/include/access/spgist.h             |  5 +-
src/include/access/spgist_private.h     |  8 +--
7 files changed, 182 insertions(+), 37 deletions(-)