Add temporal PRIMARY KEY and UNIQUE constraints

Add WITHOUT OVERLAPS clause to PRIMARY KEY and UNIQUE constraints.
These are backed by GiST indexes instead of B-tree indexes, since they
are essentially exclusion constraints with = for the scalar parts of
the key and && for the temporal part.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com

contrib/btree_gist/sql/without_overlaps.sql

@@ -0,0 +1,25 @@
+-- Core must test WITHOUT OVERLAPS
+-- with an int4range + daterange,
+-- so here we do some simple tests
+-- to make sure int + daterange works too,
+-- since that is the expected use-case.
+CREATE TABLE temporal_rng (
+  id integer,
+  valid_at daterange,
+  CONSTRAINT temporal_rng_pk PRIMARY KEY (id, valid_at WITHOUT OVERLAPS)
+);
+\d temporal_rng
+SELECT pg_get_constraintdef(oid) FROM pg_constraint WHERE conname = 'temporal_rng_pk';
+SELECT pg_get_indexdef(conindid, 0, true) FROM pg_constraint WHERE conname = 'temporal_rng_pk';
+
+INSERT INTO temporal_rng VALUES
+  (1, '[2000-01-01,2001-01-01)');
+-- same key, doesn't overlap:
+INSERT INTO temporal_rng VALUES
+  (1, '[2001-01-01,2002-01-01)');
+-- overlaps but different key:
+INSERT INTO temporal_rng VALUES
+  (2, '[2000-01-01,2001-01-01)');
+-- should fail:
+INSERT INTO temporal_rng VALUES
+  (1, '[2000-06-01,2001-01-01)');