Use size of static types in expressions

[senier]

To avoid redundancies it may be helpful to refer to the bit size of a static type in an expression, e.g. to check the size field in ARP:

   type IPv4 is
      message
         Hrd : Hardware_Address_Space
            then Pro if Hrd = Hrd_Ethernet;
         Pro : Ethernet.Ether_Type;
         HLn : Address_Length
            then PLn if 8 * HLn = Ethernet.Address'Size; -- need to add a constant 48 here now
         PLn : Address_Length
            then Op if 8 * PLn = IPv4.Address'Size; -- need to add a constant 32 here now
         Op  : Opcode;
         SHA : Ethernet.Address;
         SPA : IPv4.Address;
         THA : Ethernet.Address;
         TPA : IPv4.Address;
      end message;