[ECDSA] Add toEthSignedMessageHash

[nrryuya]

From @vignesh-msundaram.

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I also have been trying to use your ecrecover implementation from here : https://github.com/LayerXcom/verified-vyper-contracts/blob/master/contracts/ecdsa/ECDSA.vy

Since I'm using web3.eth.sign(), I realized the ecrecover didn't return the correct signer address.
So, we need to replace Line 23 :

ecrecover(_hash, convert(v, uint256), r, s)

with

ecrecover(sha3(concat("\x19Ethereum Signed Message:\n32", _hash)), convert(v, uint256), r, s)

Also, it's better to let user send r,s,v from web 3 using web3._extend.utils.toBigNumber() instead of parsing the bytes[65] input.

Eg,
In web3,

let _signature = web3.eth.sign(_signer, _message)
_signature = _signature.substr(2)
let _v = _signature.slice(128, 130) === '00' ? web3.utils.toBigNumber(27) : web3.utils.toBigNumber(28)
let _r = web3._extend.utils.toBigNumber("0x"+_signature.slice(0, 64))
let _s = web3._extend.utils.toBigNumber("0x"+_signature.slice(64, 128))
console.log(await contract.VerifySignature(_message, _v, _r, _s) === _signer)

Then in contract,

def VerifySignature(_message: bytes32, _sig_data: uint256[3]):
    return ecrecover(sha3(concat("\x19Ethereum Signed Message:\n32", _message)), _sig_data[0], _sig_data[0], _sig_data[0])

Reference : https://ethereum.stackexchange.com/questions/15364/ecrecover-from-geth-and-web3-eth-sign

[nrryuya]

About the prefix \x19Ethereum Signed Message:\n32, we can consider adding this function.

https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/cryptography/ECDSA.sol#L54