Add a cohttp-curl-async executable to fetch URLs (not installed yet)
Add `Strings` representation to `Cohttp.Body` to efficiency hold a
list of body chunks.
Add `is_empty` function to the `Cohttp.S.Body` module type.
Move flushing logic for HTTP bodies into the portable `Request` and
`Response` modules instead of individual Lwt and Async backends
plumb through the ?flush arguments to the Req/Res write function too
Update Async backend for the new Cohttp APIs that add Strings and flushing
update Lwt API for flush and Strings API changes
Address @rgrinberg review comments for 5c2212753f6328fb02bcdf077d08ec1c0c140ce7
more `>>=` to `>>|`
improve error messages from loopback test
bump date on 0.12.0 release to retag
Reduce thread allocation by replacing `return <const>` with `return_none`,
`return_unit` or `return_nil`.
Make async test terminate
Merge pull request #197 from avsm/cohttp-0.12-tweaks

Cohttp 0.12 tweaks