Refactor type_traits (API Change):

fix #373

* concepts.hpp is renamed to type_traits.hpp

* Body reader and writer concepts are renamed

include/beast/http/impl/message.ipp

@@ -10,7 +10,7 @@
 
 #include <beast/core/error.hpp>
 #include <beast/core/string_view.hpp>
-#include <beast/http/concepts.hpp>
+#include <beast/http/type_traits.hpp>
 #include <beast/http/rfc7230.hpp>
 #include <beast/core/detail/ci_char_traits.hpp>
 #include <beast/core/detail/type_traits.hpp>
@@ -130,7 +130,7 @@ prepare_content_length(prepare_info& pi,
     message<isRequest, Body, Fields> const& msg,
         std::true_type)
 {
-    typename Body::writer w(msg);
+    typename Body::reader w{msg};
     // VFALCO This is a design problem!
     error_code ec;
     w.init(ec);
@@ -159,15 +159,13 @@ prepare(message<isRequest, Body, Fields>& msg,
     Options&&... options)
 {
     // VFALCO TODO
-    static_assert(is_Body<Body>::value,
+    static_assert(is_body<Body>::value,
         "Body requirements not met");
-    static_assert(has_writer<Body>::value,
-        "Body has no writer");
-    static_assert(is_Writer<Body>::value,
-            "Writer requirements not met");
+    static_assert(is_body_reader<Body>::value,
+        "BodyReader requirements not met");
     detail::prepare_info pi;
     detail::prepare_content_length(pi, msg,
-        detail::has_content_length<typename Body::writer>{});
+        detail::has_content_length<typename Body::reader>{});
     detail::prepare_options(pi, msg,
         std::forward<Options>(options)...);