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 *)