Allow reinitializer versions greater than 256

[frangio]

To maintain storage compatibility with the original Initializable, we made reinitializer versions of type uint8. We could use a larger type here once we break compatibility. It still makes sense to pack the value with the initializing boolean, so it can't be more than uint248, but even that sounds like an exaggeration and we might want to make it a smaller value like uint64 for the possibility of adding some other values in the same storage slot in the future.

Suggested in #3870.