Improve performance of Data.ByteString.unlines

[Bodigrim]

Ideally unlines should be implemented in a style similar to intercalate, avoiding concat (which is quite ineffective):

bytestring/Data/ByteString.hs

Lines 1209 to 1227 in 5efd6b5

	intercalate :: ByteString -> [ByteString] -> ByteString
	intercalate _ [] = mempty
	intercalate _ [x] = x -- This branch exists for laziness, not speed
	intercalate (BS fSepPtr sepLen) (BS fhPtr hLen : t) =
	unsafeCreate totalLen $ \dstPtr0 ->
	unsafeWithForeignPtr fSepPtr $ \sepPtr -> do
	unsafeWithForeignPtr fhPtr $ \hPtr ->
	memcpy dstPtr0 hPtr hLen
	let go _ [] = pure ()
	go dstPtr (BS fChunkPtr chunkLen : chunks) = do
	memcpy dstPtr sepPtr sepLen
	let destPtr' = dstPtr `plusPtr` sepLen
	unsafeWithForeignPtr fChunkPtr $ \chunkPtr ->
	memcpy destPtr' chunkPtr chunkLen
	go (destPtr' `plusPtr` chunkLen) chunks
	go (dstPtr0 `plusPtr` hLen) t
	where
	totalLen = List.foldl' (\acc chunk -> acc +! sepLen +! length chunk) hLen t
	(+!) = checkedAdd "intercalate"

Originally posted by @Bodigrim in #477 (comment)