Inconsistency with the standard for `burn` and `operatorBurn`

[daejunpark]

There is a discrepancy between the standard and the reference implementation regarding the burn and operatorBurn functions:

The current standard (https://github.com/ethereum/EIPs/blob/b41669ede219b9599a6a1c637e578b8a444ad6b1/EIPS/eip-777.md) says:

The data field of the tokensToSend hook MUST be empty.

But, the current reference implementation doesn't check (or enforce) the emptiness (of _holderData):

https://github.com/jacquesd/ERC777/blob/896d077eb4b1beb0fc2258f9d4dff4b60478a479/contracts/ERC777BaseToken.sol#L202

[tomtomjhj]

plus, the signatures for burn and operatorBurn are not updated.
https://github.com/ethereum/EIPs/blame/b41669ede219b9599a6a1c637e578b8a444ad6b1/EIPS/eip-777.md#L57-L58
https://github.com/ethereum/EIPs/blame/b41669ede219b9599a6a1c637e578b8a444ad6b1/EIPS/eip-777.md#L488
https://github.com/ethereum/EIPs/blame/b41669ede219b9599a6a1c637e578b8a444ad6b1/EIPS/eip-777.md#L501