Tidy up core sources:

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

include/beast/http/string_body.hpp

@@ -9,8 +9,8 @@
 #define BEAST_HTTP_STRING_BODY_HPP
 
 #include <beast/http/body_type.hpp>
-#include <beast/buffer_cat.hpp>
-#include <beast/streambuf.hpp>
+#include <beast/core/buffer_cat.hpp>
+#include <beast/core/streambuf.hpp>
 #include <memory>
 #include <string>