struct IncomingMessageHandleStorage

|IncomingMessageHandleStorage| allocates the necessary storage for receiving

the handles for a transactional message of type |FidlType| from its

corresponding transport. It is useful for allocating bespoke amount of memory

when the code path is certain what is the message type, such as during

synchronous calls.

|FidlType| must be a specialization of |TransactionalResponse|.

This class is shared between wire and natural types - both receive messages

of the same shapes and then perform their own decoding.