read_size replaces read_size_helper:

* detail::read_size_helper is removed

* public function read_size helps determine the optimum
  size for calls to a DynamicBuffer's prepare function.

* special case for Asio types which don't conform to
  the concept but have their own helper function.

test/Jamfile

@@ -34,6 +34,7 @@ unit-test core-tests :
     core/handler_ptr.cpp
     core/multi_buffer.cpp
     core/ostream.cpp
+    core/read_size.cpp
     core/static_buffer.cpp
     core/static_string.cpp
     core/string_param.cpp