Expose more low-level interfaces for HTTP body handling

Bring make_body_writer and write_header into Cohttp.S.Http_io. Needed by ocaml-git

Support Lwt 2.5.0 and assorted interface cleanups

Compatibility breaking interface changes:

• Remove read_form from the Request/Response/Header interfaces
  as this should be done in Body handling instead (#401).

New features and bug fixes:

• Remove IO.write_line as it was unused in any interfaces.
• Do not use the lwt camlp4 extension. No observable external difference.
• Do not return a code stacktrace in the default 500 handler.
• Add Cohttp.Header.compare (#411)
• Fix typos in CLI documentation (#413 via @moonlightdrive)
• Use the Lwt 2.5.0 buffer API.
• Cohttp_lwt.read_response now has a non-optional closefn parameter (#400).
• Add a Cohttp_lwt_s module that contains all the Lwt module types
  in one convenient place (#397).

HTTP pipelining and DELETE improvements

• Allow DELETE requests to have request bodies (#383).
• Improve the Lwt client callv for HTTP/1.1 pipelined
  requests (#379 via Török Edwin).

Fix 204 code response headers

• Do not add content encoding for 204's (#375)

HTTP header parsing bug fixes

• Remove trailing whitespace from headers (#372)
• Don't reverse order of list valued headers (#372)

Top level printers, Async callv, bug fixes

• Add Cohttp_async.Client.callv. Allows for making requests while reusing an
  HTTP connection (#344)
• Responses of status 1xx/204/304 have no bodies and cohttp should not attempt
  to read them (#355)
• Add top level printers. See cohttp.top findlib package (#363)
• Add Header.to_string (#362)
• Fix chunk truncation in chunked transfer encoding (#360)

Compatibility breaking interface changes:

• Remove Request/Response modules outside of Cohttp pack (#349)

Much better handling of large fixed size bodies

• Remove dependency on the Lwt Camlp4 syntax extension (#334).
• Add make github target to push documentation to GitHub Pages
  (#338 from Jyotsna Prakash).
• [async] Add Cohttp_async.Server.close to shutdown server (#337).
• Add Async integration tests and consolidate Lwt tests using the
  new framework (#337).
• Fix allocation of massive buffer when handling fixed size http bodies (#345)

Improved Async buffer handling

• [async] Limit buffer size to a maximum of 32K in the Async backend
  (#330 from Stanislav Artemkin).
• Add Cohttp.Conf.version with the library version number included.
• Remove debug output from cohttp-curl-async.
• Add the beginning of a DESIGN.md document to explain the library structure.

Support more HTTP methods, Link support and stability improvements

Compatibility breaking interface changes:

• CONNECT and TRACE methods added to Code.Exhaustive matches will need updating.

New features and bug fixes:

• Link header parsing has been added as Cohttp.Link, Header.get_links and Header.add_links
• cohttp_server_* now obeys HEAD requests and responds 405 to unknown methods
• Cohttp_async.Server.response type is now exposed as a response * body pair
• Failure to read a body in a pipelined response no longer terminates the stream
• Fix cohttp_curl_lwt -X HEAD sending empty chunked body (#313)
• Fix a bug which left extra \r\n in buffer at end of chunked reads
• Fix handling of request URI for query strings and CONNECT proxies (#308, #318)
• Fix precedence of Host header when request-URI is absolute URI
• Fix request URI path to be non-empty except for * requests (e.g. OPTIONS *)

Fix Uri Handling

New features and bug fixes:

• Fix handling of request paths starting with multiple slashes (#308)