Shortcut for pinned bytearray

[Bodigrim]

Resolves #186. Cf. #253. CC @chessai @andrewthad.

I'm reluctant to provide unsafeByteArrayToByteString in a public interface, but we can add a shortcut to create ByteString from a pinned ByteArray#. Together with #313, this gives us an efficient way to convert between ByteArray and ByteString without reaching for internal modules:

import Data.ByteString.Short (ShortByteString(..), fromShort, toShort)

byteArrayToByteString :: ByteArray -> ByteString
byteArrayToByteString (ByteArray b#) = fromShort (SBS b#)

byteStringToByteArray :: ByteString -> ByteArray
byteStringToByteArray bs = let SBS b# = toShort bs in ByteArray b#

[andrewthad]

This looks correct to me.

[chessai]

LGTM