Footer after field with size defined by message size

[treiher]

It should be possible to have a footer after a field, which size is defined by the message size.

package Test is

   type T is mod 2**8;

   type M is
      message
         A : T;
         B : Opaque
            with Size => Message'Size - 2 * Test::T'Size;
         C : T;
      end message;

end Test;

test.rflx:8:10: model: error: negative size for field "B" (A -> B)

[treiher]

The use case can be realized using parameterized messages. The issue will be obsolete after realization of #736 , as the use of Message'Size in size aspects will be prohibited. At the moment, it is not planned to support the use case using Message'Size.