Make Bitmapsets be valid Nodes.

Add a NodeTag field to struct Bitmapset.  This is free because of
alignment considerations on 64-bit hardware.  While it adds some
space on 32-bit machines, we aren't optimizing for that case anymore.
The advantage is that data structures such as Lists of Bitmapsets
are now first-class objects to the Node infrastructure, and don't
require special-case code to handle.

This patch includes removal of one such special case, in indxpath.c:
bms_equal_any() can now be replaced by list_member().  There may be
more existing code that could be simplified, but I didn't look very
hard.  We also get to drop the read_write_ignore annotations on a
couple of RelOptInfo fields.

The outfuncs/readfuncs support is arranged so that nothing changes
in the string representation of a Bitmapset field; therefore, this
doesn't need a catversion bump.

Amit Langote and Tom Lane

Discussion: https://postgr.es/m/109089.1668197158@sss.pgh.pa.us

src/backend/nodes/outfuncs.c

@@ -314,6 +314,9 @@ _outList(StringInfo str, const List *node)
  *	   converts a bitmap set of integers
  *
  * Note: the output format is "(b int int ...)", similar to an integer List.
+ *
+ * We export this function for use by extensions that define extensible nodes.
+ * That's somewhat historical, though, because calling outNode() will work.
  */
 void
 outBitmapset(StringInfo str, const Bitmapset *bms)
@@ -844,6 +847,8 @@ outNode(StringInfo str, const void *obj)
 		_outString(str, (String *) obj);
 	else if (IsA(obj, BitString))
 		_outBitString(str, (BitString *) obj);
+	else if (IsA(obj, Bitmapset))
+		outBitmapset(str, (Bitmapset *) obj);
 	else
 	{
 		appendStringInfoChar(str, '{');