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

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