Decrease `ptr_uint64::max_label` to support larger decision diagrams

[SSoelvsten]

Currently, the ptr_uint64 class has the number of bits for the label set to 24 (similar as Sylvan). Yet, after the change #475, we have removed another bit from the level index. This decreases the maximum level size to 3 TiB and the largest BDD at 6 TiB before possibly breaking due to overflow errors.

Choices:

In ascending order of number of bits:

1. If we fix the number of variables to be 16 bits, i.e. it fits into a uint16_t, then the compiler can give overflow warnings. This might be welcomed by the end-user.

2. By default, the maximum file size on Linux (with ext4) is 16 TiB. We should at least support a BDD of this size, i.e. where the largest level is up to 8 TiB wide. Hence, we would want to use 39 bits for the level (12 TiB).

3. In fact, considering #154, we may want to support each level reaching the maximum file size. This would require one more bit, i.e. the id needs 40 bits. Yet, at this point you have a single BDD of at least 32 TiB. Is this even realistic?

Other Tasks:

• [x] We can almost regain an entire bit for the label by merging the terminal_flag with the label bits. That is, we make the maximum level NIL and the next-most level terminal. This decreases the number of BDD labels by two to then bring back the number of ids to 6 TiB. (done in #591)
• [ ] Maybe we should (also in production) check for a per-level overflow in non-debug mode and throw an exception.
• [ ] By decreasing it by one more bit than described above, we future-proof for QMDDs ( #443 ). Or, better yet is to flip the derivation of the number of bits, such that both QMDDs and BDDs use the same number of id bits, but BDDs have one more label bit. If we decrease the label to 16 bits, then this should be skipped.

Additional Context:

For reference, here are the upper bounds in other BDD packages

BDD Package	Max Variable	Source
BuDDy	221 + 1 - 1	src/buddy/kernel.h #81
CAL	216 + 1 - 2	src/calInt.h #200
CUDD	232 (or 16) + 1 -1	cudd/cuddInt.h #205, #261
JBDD	217 + 1 - 1	src/main/java/de/tum/in/jbdd/BddImpl.java #L68
Lib-BDD	216 + 1 - 1	src/_impl_bdd_variable.rs
OxiDD	232 + 1 - 1	oxidd/crates/oxidd-core/src/lib.rs
PJBDD	232 + 1 - 1	src/org/sosy_lab/pjbdd/bdd/bddNode.java #41
Sylvan	224 + 1 -1	src/sylvan_mtbdd_int.h #L117

[SSoelvsten]

Asking Randy Bryant, Jaco van de Pol, and Anna Blume Jakobsen, they all confirm we can decrease the number of labels with a few bits.

[SSoelvsten]

With #591 we already brought the maximum safe BDD size back up 12 TiB; this seems out of reach for most (and even our own experiments). Hence, I will just close it for now.