tighten bounds on bytestring

[vmchale]

At the moment it fails with

Codec/Compression/Snappy/Lazy.hsc:55:15: error:
    Ambiguous occurrence ‘BS’
    It could refer to
       either ‘Data.ByteString.Internal.BS’,
              imported from ‘Data.ByteString.Internal’ at Codec/Compression/Snappy/Lazy.hsc:34:1-51
           or ‘Codec.Compression.Snappy.Lazy.BS’,
              defined at Codec/Compression/Snappy/Lazy.hsc:48:14
   |
55 |     poke ptr (BS (PS fp off len)) = withForeignPtr fp $ \p -> do
   |               ^^

Codec/Compression/Snappy/Lazy.hsc:98:23: error:
    Ambiguous occurrence ‘BS’
    It could refer to
       either ‘Data.ByteString.Internal.BS’,
              imported from ‘Data.ByteString.Internal’ at Codec/Compression/Snappy/Lazy.hsc:34:1-51
           or ‘Codec.Compression.Snappy.Lazy.BS’,
              defined at Codec/Compression/Snappy/Lazy.hsc:48:14
   |
98 |   r <- withArray (map BS chunks) $ \chunkPtr ->
   |                       ^^

which was introduced in bytestring 0.11.0.0

Thanks!