pgsql: Allow subscripting of hstore values.

Allow subscripting of hstore values.

This is basically a finger exercise to prove that it's possible for
an extension module to add subscripting ability.  Subscripted fetch
from an hstore is not different from the existing "hstore -> text"
operator.  Subscripted update does seem to be a little easier to
use than the traditional update method using hstore concatenation,
but it's not a fundamentally new ability.

However, there may be some value in the code as sample code, since
it shows what's basically the minimum-complexity way to implement
subscripting when one needn't consider nested container objects.

Discussion: https://postgr.es/m/3724341.1607551174@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0ec5f7e78231a621a1d96c4bfedc4a1849a6c6cc

Modified Files
--------------
contrib/hstore/Makefile             |   4 +-
contrib/hstore/expected/hstore.out  |  23 +++
contrib/hstore/hstore--1.7--1.8.sql |  13 ++
contrib/hstore/hstore.control       |   2 +-
contrib/hstore/hstore_subs.c        | 297 ++++++++++++++++++++++++++++++++++++
contrib/hstore/sql/hstore.sql       |   8 +
doc/src/sgml/hstore.sgml            |  42 +++++
doc/src/sgml/ref/create_type.sgml   |   8 +-
8 files changed, 392 insertions(+), 5 deletions(-)