pgsql: Add reusable routine for making arrays unique.

Add reusable routine for making arrays unique.

Introduce qunique() and qunique_arg(), which can be used after qsort()
and qsort_arg() respectively to remove duplicate values.  Use it where
appropriate.

Author: Thomas Munro
Reviewed-by: Tom Lane (in an earlier version)
Discussion: https://postgr.es/m/CAEepm%3D2vmFTNpAmwbGGD2WaryM6T3hSDVKQPfUwjdD_5XY6vAA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7815e7efdb4ce9575b5d8460beb0dd2569d7ca3a

Modified Files
--------------
contrib/hstore/hstore_io.c           |  5 +++
contrib/intarray/_int_tool.c         | 19 +++--------
contrib/pg_trgm/trgm_op.c            | 25 ++------------
src/backend/access/nbtree/nbtutils.c | 19 ++---------
src/backend/executor/nodeTidscan.c   | 13 ++------
src/backend/utils/adt/acl.c          | 15 +++------
src/backend/utils/adt/tsgistidx.c    | 29 ++--------------
src/backend/utils/adt/tsquery_op.c   | 29 +++-------------
src/backend/utils/adt/tsvector.c     |  5 +--
src/backend/utils/adt/tsvector_op.c  | 59 +++++---------------------------
src/backend/utils/adt/txid.c         | 19 ++---------
src/backend/utils/cache/syscache.c   | 21 ++++--------
src/include/lib/qunique.h            | 65 ++++++++++++++++++++++++++++++++++++
13 files changed, 115 insertions(+), 208 deletions(-)