Change nodetable and cache indexing from `int` to `ssize_t`

[ssoelvsten]

The current implementation has the unique node table in kernel.h and the operations cache in cache.h. From bdd.h one can see, that the bdd variable exposed to the user really just is an index into the unique node table.

So, if we replace the int types with a ssize_t (signed size type), then we can expand the BuDDy library to make use of 64-bit indexation.

• Use the BDD typedef more thoroughly throughout the code-base.
• Add the BUDDY_64_BIT pre-processing variable. If set then ssize_t is defined as signed long long and size_t as unsigned long long. If not, then ssize_t is only a signed long and size_t an unsigned long.
• Change BDD to be a ssize_t