Add flat_streambuf:

Objects of this type meet the requirements of DynamicBuffer
and offer an additional invariant: buffer sequences returned
by data() and prepare() are always of length one.

include/beast/core.hpp

@@ -17,6 +17,7 @@
 #include <beast/core/buffers_adapter.hpp>
 #include <beast/core/consuming_buffers.hpp>
 #include <beast/core/error.hpp>
+#include <beast/core/flat_streambuf.hpp>
 #include <beast/core/handler_alloc.hpp>
 #include <beast/core/handler_concepts.hpp>
 #include <beast/core/handler_helpers.hpp>