I put the issues into an PR so it's simpler to describe it in code: #53
1 (ERC3525) burning nfts implicitly decreases token supply (that's ok but maybe worth another thought)
2 (Enumerable) tokenInSlotByIndex / tokenSupplyInSlot yield wrong values after a value transfer
3 (merging) totalSupply doesn't decrease after merging values.
All details inside the PR's description :)