Add StorableConduit benchmark.

Hah, looks like this entire package is unnecessary! I hadn't
realized/remembered that storable vectors also use ForeignPtrs, just
like ByteString. So all we need is a pair of helper functions to convert
between the two representations. One direction of that conversion is:

fromStorableVector :: V.Vector Word8 -> S.ByteString
fromStorableVector v =
    PS fptr offset idx
  where
    (fptr, offset, idx) = V.unsafeToForeignPtr v

Pinging @chrisdone, since we discussed this code yesterday.