Allow with-clause without then-clause to define field length

[treiher]

Being able to directly define the field length at the specific field could simplify the specification in cases, where the length of the field does not depend on any condition, but multiple paths exist (i.e. multiple incoming edges contain the same length aspect). Such a length specification should be internally translated into length aspects at all incoming edges. No incoming edge should be allowed to have a length aspect in such a case. In #95 we already thought about introducing a similar feature for conditions.

Examples

TLS

         Verify_Data : Opaque
            with Length => Message'Length;

Instead of:

         null
            then Verify_Data
               with Length => Message'Length;
         Verify_Data : Opaque;

IPv4

         [...]
         Destination : Address
            then Payload
               if IHL = 5,
            then Options
               with Length => IHL * 32 - (5 * 32)
               if IHL > 5;
         Options : Options;
         Payload : Opaque
            with Length => Total_Length * 8 - (IHL * 32);

Instead of:

         [...]
         Destination : Address
            then Payload
               with Length => Total_Length * 8 - (IHL * 32)
               if IHL = 5,
            then Options
               with Length => IHL * 32 - (5 * 32)
               if IHL > 5;
         Options : Options
            then Payload
               with Length => Total_Length * 8 - (IHL * 32);
         Payload : Opaque;