Implement compareLength for lazy bytestrings

[Bodigrim]

It would be nice to have an implementation of

compareLength :: Data.ByteString.Lazy.ByteString -> Int -> Ordering

One could write compareLength xs n = compare (length xs) n, but this necessarily forces xs in full, which is undesirable in many use cases. We are looking for a more lazy implementation, similar in properties to Data.Text.compareLength.

[sjakobi]

Once we have an implementation, we can also add RULES similar to those in text: http://hackage.haskell.org/package/text-1.2.4.0/docs/src/Data.Text.html#compareLength