to squash delicate code duplication and for speed.

+ This is optimized in several subtle ways over the current method.  This
  is documented in Maintainer.txt, along with a correctness proof.

+ I'll replace all the BTree searches with this soon.  For now I just
  changed _BTree_get().  I *suspect* this also fixes a subtle bug
  introduced by the recent "don't ignore comparison error" patch:  if a
  comparison did trigger an exception, the function just returned without
  doing the PER_ALLOW_DEACTIVATION + PER_ACCESSED dance.  _BTree_get()
  does that dance again now.