Re: Revise the Asserts added to bimapset manipulation functions

I've now adjusted the patch to have all modifications to Bitmapsets
return a newly allocated set. There are a few cases missing in master
that need to be fixed (items 6-10 below):

The patch now includes changes for the following:

1. Document what REALLOCATE_BITMAPSETS is for.
2. Provide a reusable function to check a set is valid and use it in
cassert builds and use it to validate sets (Richard)
3. Provide a reusable function to copy a set and pfree the original
and use that instead of duplicating that code all over bitmapset.c
4. Fix Assert duplication (Richard)
5. Make comments in bms_union, bms_intersect, bms_difference clear
that a new set is allocated.  (This has annoyed me for a while)
6. Fix failure to duplicate the set in bms_add_members() when b == NULL.
7. Fix failure to duplicate the set in bms_add_range() when upper < lower
8. Fix failure to duplicate the set in bms_add_range() when the set
has enough words already.
9. Fix failure to duplicate the set in bms_del_members() when b == NULL
10. Fix failure to duplicate the set in bms_join() when a == NULL and
also fix the b == NULL case too
11. Fix hazard in bms_add_members(), bms_int_members(),
bms_del_members() and bms_join(),  where the code added in 7d58f2342
could crash if a == b due to the REALLOCATE_BITMAPSETS code pfreeing
'a'. This happens in knapsack.c:93, although I propose we adjust that
code now that empty sets are always represented as NULL.