* src/bddop.c, src/bddx.h, src/cache.c, src/cache.h, src/kernel.c,
src/kernel.h, src/prime.c, src/prime.h, src/reorder.c: Use power of
two for the sizes of all hash tables, in order to reduce the amount of
divisions performed. Also allow bddhash to be smaller than bddnodes.
The double result is never used with a triple keys,
so we can pack the cache entry more tightly.
* src/cache.h: Reorganize the cache entry the structure.
* src/cache.c: Cleanup the code while we are at it.
* src/bddop.c: Adjust to accesses to cache entries.
