Graph implementation for low-arity nodes

[viking-sudo-rm]

Many nodes in the CDAWG likely have low arity. It could be possible to use a different edge representation for these nodes (e.g., linked list or array instead of tree), and save a lot of memory.

[viking-sudo-rm]

The memory overhead improvements from this change should be strictly less than converting to an immutable CDAWG (though this would preserve mutability). For this reason, this is deprioritized.