Implemented the Retriever.
Unfortunately, it is a fileRetriever and not a byteRetriever.
It should be possible to convert this to a byteRetiever, but I got stuck:
The conduit sink needs to process individual chunks, but a byteRetriever
needs to pass a single L.ByteString to its callback for processing. I
looked into using unsafeInerlaveIO to build up the bytestring lazily,
but the sink is already operating under conduit's inversion of control,
and does not run directly in IO anyway.
On the plus side, no more memory leak..
The remaining leaks are in hS3. The leak with encryption was worked around
by the use of the temp file. (And was probably originally caused by
gpgCipherHandle sparking a thread which kept a reference to the start
of the byte string.)
