Use bytes.concat in MessageHashUtils

[frangio]

We can use bytes.concat instead of abi.encodePacked here which is clearer.

Updating an existing changeset to mention this PR too.

In toDataWithIntendedValidatorHash we keep abi.encodePacked because one of the arguments is an address.

[changeset-bot]

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[Amxx]

FYI, I considered that change in #4296, but was not sure because of the casting.

both

keccak256(bytes.concat("\x19Ethereum Signed Message:\n", bytes(Strings.toString(message.length)), message));

and

keccak256(string.concat("\x19Ethereum Signed Message:\n", Strings.toString(message.length), string(message)));

should work. Any reason to favor the first one ?

[frangio]

keccak256 requires a bytes argument.