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.

test/Jamfile

@@ -25,6 +25,7 @@ unit-test core-tests :
     core/consuming_buffers.cpp
     core/dynabuf_readstream.cpp
     core/error.cpp
+    core/flat_streambuf.cpp
     core/handler_alloc.cpp
     core/handler_concepts.cpp
     core/handler_ptr.cpp