Add Eq instance for Data.Attoparsec.ByteString.Lazy.Result

An `Eq` instance would be useful e.g. for parser unit tests, testing not only the result of the parse but that the expected amount of left-over input is present.

This would only be the for the lazy `Result` type because `IResult` can contain a function:
```
Partial :: (i -> IResult i r) -> IResult i r
```