ERC-777 token support about v1-contracts

[MicahZoltu]

While I disagree that tokens that call into the recipient are inherently unsafe, I don't think that is relevant here since Uniswap cannot dictate how tokens function internally. Since Uniswap supports arbitrary tokens (one of its biggest selling points!), I think it needs to assume the worst and that the token will do all of the worst things possible when called.

[haydenadams]

More broadly speaking, the Uniswap contracts incorrectly assume that the token contract being called will not call out into any third party

Imo tokens that do this are inherently unsafe and should not be used.

That said, mutex has already been added in the v2 branch
https://github.com/Uniswap/contracts-vyper/blob/v2/contracts/uniswap_exchange.vy#L66

Hopefully EIP 2200 makes it into the Istanbul (final decision this friday) so its not a huge gas hit.