Refactor header status, reason, and target (API Change):

* header::result is a family of functions to replace header::status

* header interface now uses status enum and also ints

* reason-phrase is no longer stored unless the user explicitly
  requests it.

* When serializing, the standard reason is used for the
  corresponding status code unless the user has changed it.

test/http/test_parser.hpp

@@ -62,11 +62,11 @@ public:
     }
 
     void
-    on_response(int status_,
+    on_response(int code,
         string_view reason_,
             int version_, error_code& ec)
     {
-        status = status_;
+        status = code;
         reason = std::string(
             reason_.data(), reason_.size());
         version = version_;