[PATCH v2 0/2] Small clean-ups to the doc ID set code

[Austin Clements]

I split up the patches as requested and also rebased to current
master.  Relative to the original patch, I stripped out the
bound-to-max change since I could no longer convince myself it was a
good idea, and changed BITMAP_* to DOCIDSET_* to clarify that the
macros were for working with the doc ID set bitmaps and not just any
old bitmaps.

As usual, this is also available in my git repo
  http://awakening.csail.mit.edu/git/notmuch.git
as the search-perf-3 branch.