Use a safe technique to implement `.get2_mut()`

[CosmicHorrorDev]

Fixes #41

Uses the same technique that slab uses (as mentioned in https://github.com/LPGhatguy/thunderdome/issues/41#issuecomment-1487614744) with some slight tweaks to handle the generations right

[CosmicHorrorDev]

Thanks for all the cleanup! It looks a lot cleaner now :)