EIP-4521: 721/20-compatible transfer

This EIP provides references to add such a transfer() to ERC-721.

Replicating ERC-20 transfer() with just a minor change to accurately reflect that a unique tokenId rather than fungible sum is being sent is desirable for code simplicity and to make integration easier.

Full EIP: https://github.com/ethereum/EIPs/pull/4557/files