The unicity table was mixed with the bddNode table for now
apparent reason.  After the hash of some node is computed,
bddnodes[hash] did only contain some random node (not the one
looked for) whose .hash member would point to the actual node with
this hash.  So that's a two step lookup.  With this patch, we sill
have a two step lookup, but the .hash member have been moved to a
separate array.  A consequence is that bddNode is now 16-byte long
(instead of 20) so it will never span across two cache lines.

* src/kernel.h (bddNode): Remove the hash member, and move it...
(bddhash): ... as this new separate table.
* src/kernel.c, src/reorder.c: Adjust all code.