struct IncomingMessageStorage

|IncomingMessageStorage| allocates the necessary storage for receiving a

transactional message of type |FidlType| from its corresponding transport.

The allocation may be inline if reasonably small. 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.