move some bitmapset.c macros to bitmapset.h

Over in [1], Masahiko and I found that using some bitmapset logic yields a
useful speedup in one part of the proposed radix tree patch. In addition to
what's in bitmapset.h now, we'd need WORDNUM, BITNUM, RIGHTMOST_ONE and
bmw_rightmost_one_pos() from bitmapset.c. The file tidbitmap.c has its own
copies of the first two, and we could adapt that strategy again. But
instead of three files defining these, it seems like it's time to consider
moving them somewhere more central.

Attached is a simple form of this concept, including moving
HAS_MULTIPLE_ONES just for consistency. One possible objection is the
possibility of namespace clash. Thoughts?

I also considered putting the macros and typedefs in pg_bitutils.h. One
organizational advantage is that pg_bitutils.h already offers convenience
function symbols where the parameter depends on SIZEOF_SIZE_T, so putting
the BITS_PER_BITMAPWORD versions there makes sense. But that way is not a
clear win, so I didn't go that far.

[1]
https://www.postgresql.org/message-id/CAFBsxsHgP5LP9q%3DRq_3Q2S6Oyut67z%2BVpemux%2BKseSPXhJF7sg%40mail.gmail.com

--
John Naylor
EDB: http://www.enterprisedb.com