Custom instances of HStream impossible to write

It would be nice to be able to have an instance of [`HStream`](http://hackage.haskell.org/package/HTTP-4000.2.19/docs/Network-TCP.html#t:HStream) that allows to stream without using lazy I/O; in particular, an instance that would allow us to close the connection when we no longer want the rest of the response body.

However, in order to define a custom type with a corresponding `HStream` instance one needs to provide, amongst other things, implementations of

``` haskell
openStream :: String -> Int -> IO (HandleStream bufType)
close :: HandleStream bufType -> IO ()
```

For the implementation of `openStream` we can use

``` haskell
openTCPConnection :: BufferType ty => String -> Int -> IO (HandleStream ty)
```

However, for the implementation of `close` we are stuck; there is no function analogous to `openTCPConnection` that allows us to close a `HandleStream`.

Although we have

``` haskell
hstreamToConnection :: HandleStream String -> Connection
```

and there is a [`Stream`](http://hackage.haskell.org/package/HTTP-4000.2.19/docs/Network-Stream.html#t:Stream) instance for [`Connection`](http://hackage.haskell.org/package/HTTP-4000.2.19/docs/Network-TCP.html#t:Connection), `hstreamToConnection` works _only_ for `HandleStream String`, so that doesn't help either.

Finally, the [`HandleStream`](http://hackage.haskell.org/package/HTTP-4000.2.19/docs/Network-TCP.html#t:HandleStream) type is completely opaque (it does not even satisfy `Functor`) so we cannot define a `HStream` instance for one `bufType` in terms of the `HStream` instance for another. If we had _some_ way of defining new instances in terms of old instances (for instance, reuse the instance for lazy bytestrings, but somehow retain a handle to the corresponding `HandleStream` so that we can close it explicitly) that would be great.