Use the latest ERC-721 draft interface, fixes #745

[fulldecent]

Fixes #745

🚀 Description

The ERC-721 is near finalization. I have included the latest interfaces into here.

We are now working on interop testing and would love to work with the OpenZeppelin project on this.

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented. HELL YES
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[fulldecent]

Quoting @spalladino from https://gitter.im/ETH-NFT/Lobby to move the discussion here.

For minor discussion on #777, would you rather keep the discussion here, or on the PR itself? For instance, I'd consider changing visibility of external methods to public (as in ERC20), or remove seemingly unnecessary payable modifiers.

[fulldecent]

I have made these requested changes.

Just wanted to note here that it is okay that OpenZeppelin's interface for ERC-721 is different than the canonical ERC-721 (external -> public, no payable). And it does not mean that the standard is deficient. The standard recognizes and makes allowance for this already.

Specifically, the standard is correct for the long term, future versions of Solidity. And this file in OZ is correct today.

Citing https://github.com/fulldecent/EIPs/blob/patch-2/EIPS/eip-721.md#caveats:

• Interface functions are external, or no interfaces ethereum/solidity#2330
  • Current versions of Solidity allow using public in an interface and disallow using external in an interface with public in a contract.
  • Future Solidity requires external in interfaces and allows contracts with public to implement those functions.
• Inheritance: allow stricter mutability ethereum/solidity#3412:
  • Current versions of Solidity require a payable interface to be implemented by a payable contract function.
  • Future versions of Solidity will allow a payable interface to be implemented by a default non-payable contract function.

I'm pointing this out to assert that the standard does regulate on the payability and non-payability of functions. And I continue to fight to make the Solidity interface grammar more expressive!

[spalladino]

Thanks @fulldecent! I'll compare this interface with the candidate implementation from @facuspagnuolo at #682, rework the implementation as needed, and revert back here if there is any strong mismatch.

[spalladino]

Closing in favour of #803. Note that this interface was included in that PR. Again, thanks for your work @fulldecent!