Tidy up core sources:

The core headers are moved to their own directory (but remain in
the same namespace).

test/http/nodejs_parser.hpp

@@ -10,10 +10,10 @@
 
 #include "nodejs-parser/http_parser.h"
 
-#include <beast/error.hpp>
 #include <beast/http/message_v1.hpp>
 #include <beast/http/rfc2616.hpp>
-#include <beast/buffer_concepts.hpp>
+#include <beast/core/buffer_concepts.hpp>
+#include <beast/core/error.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/system/error_code.hpp>
 #include <cstdint>