Slot map: array storage with persistent indices

Vec<T>-like collection with stable indices. The underlying array's indices are re-used by incementing a versioning tag in the index type.

The SlotMapIndex type consists of a u32 for storing the index into the underlying array, and a u32 for storing the version. Deleting and inserting more times than the maximum value of u32 will cause overflow and index conflict bugs.