burning / enumeration issues

[elmariachi111]

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 :)