Teach pageinspect about nbtree deduplication.

Add a new bt_metap() column to display the metapage's allequalimage
field.  Also add three new columns to contrib/pageinspect's
bt_page_items() function:

* Add a boolean column ("dead") that displays the LP_DEAD bit value for
each non-pivot tuple.

* Add a TID column ("htid") that displays a single heap TID value for
each tuple.  This is the TID that is returned by BTreeTupleGetHeapTID(),
so comparable values are shown for pivot tuples, plain non-pivot tuples,
and posting list tuples.

* Add a TID array column ("tids") that displays TIDs from each tuple's
posting list, if any.  This works just like the "tids" column from
pageinspect's gin_leafpage_items() function.

No version bump for the pageinspect extension, since there hasn't been a
stable Postgres release since the last version bump (the last bump was
part of commit 58b4cb30).

Author: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-WzmSMmU2eNvY9+a4MNP+z02h6sa-uxZvN3un6jY02ZVBSw@mail.gmail.com

contrib/pageinspect/expected/btree.out

@@ -12,6 +12,7 @@ fastroot                | 1
 fastlevel               | 0
 oldest_xact             | 0
 last_cleanup_num_tuples | -1
+allequalimage           | t
 
 SELECT * FROM bt_page_stats('test1_a_idx', 0);
 ERROR:  block 0 is a meta page
@@ -41,6 +42,9 @@ itemlen    | 16
 nulls      | f
 vars       | f
 data       | 01 00 00 00 00 00 00 01
+dead       | f
+htid       | (0,1)
+tids       | 
 
 SELECT * FROM bt_page_items('test1_a_idx', 2);
 ERROR:  block number out of range
@@ -54,6 +58,9 @@ itemlen    | 16
 nulls      | f
 vars       | f
 data       | 01 00 00 00 00 00 00 01
+dead       | f
+htid       | (0,1)
+tids       | 
 
 SELECT * FROM bt_page_items(get_raw_page('test1_a_idx', 2));
 ERROR:  block number 2 is out of range for relation "test1_a_idx"