eta-contract Response.read_body_chunk as we already have for Request.rea...

...d_body_chunk

This allows a streaming download loop like the following:
```
  let reader = Response.read_body_chunk response in
  (* the remaining length reference cell is now captured in 'reader' *)
  let rec loop () =
    reader ic
    >>= function
    | Cohttp.Transfer.Chunk x ->
      Buffer.add_string body x;
      loop ()
    | Cohttp.Transfer.Final_chunk x  ->
      Buffer.add_string body x;
      return (Buffer.contents body)
    | Cohttp.Transfer.Done ->
      return (Buffer.contents body) in
  loop () >>= fun body ->
```
Without this, every call to reader ic generates a fresh reference cell and the
download never completes.

This mirrors the API in #154

Signed-off-by: David Scott <dave.scott@eu.citrix.com>