a BTree is empty if and only if its firstbucket pointer is
NULL, so there's no need to keep checking nonzero inside the
loop.  Also, the pointers here are all local, so there's no
need to incref and decref them -- the apparent necessity was
just an artifact of using the bucket assignment macros.  So
stopped using those macros, and the need for incref/decref
operations went away too.

PyVar_AssignB(), ASSIGNB(), ASSIGNBC():  Turned out these
are no longer used anywhere anymore, so removed them.

PER_UNUSE():  New macro to capture the endlessly repeated
PER_ALLOW_DEACTIVATION + PER_ACCESSED pair.  As the comment
says, so sue me <wink>.

Bucket_deleteNextBucket():  Documented & simplified.