Make CalculateChecksum accept []byte, not Frame

[bartekpacia]

This is just an idea. This might make the API more flexible.

It will be a breaking change, so v2.0.0 will have to be released, but currently not many projects depend on this package, so this is might be a sensible thing to do.

[TK-A369]

So, you think to calculate checksum only from data, and not from from header, length byte and plus sign? Do I understand it correctly?
If yes, I think it's not good idea, because in this case, when errors occurs while transmitting i.e. header, checksum will not change, and this communication error cannot be detected.

[bartekpacia]

No no, it's purely a syntactical change. It's hard to explain a difference to someone not familiar with Go. We will still be verifying the whole frame:)