This patch addresses the BuDDy part of #396,
reported by Florian Renkin and Reed Oei.

* src/kernel.c, src/kernel.h: Declare a bddrecstack and
associated macros.  Resize it when new variable are declared.
* src/cache.h: Add a BddCache_index macro.
* src/bddop.c (not_rec, apply_rec, quant_rec, appquant_rec,
support_rec, ite_rec, compose_rec, restrict_rec, satone_rec,
satoneset_rec): Rewrite using this stack to get rid of the recursion.