Simplify setting of opaque fields

[treiher]

Setting an opaque field is unnecessary complicated:

   Data : RFLX.RFLX_Builtin_Types.Bytes (RFLX.RFLX_Builtin_Types.Index'First .. RFLX.RFLX_Builtin_Types.Index'First + 2**14);

   function Valid_Data_Length (Length : RFLX.RFLX_Builtin_Types.Length) return Boolean is
      (Length <= Data'Length);

   procedure Write_Data (Buffer : out RFLX.RFLX_Builtin_Types.Bytes) with
      Pre => Valid_Data_Length (Buffer'Length)
   is
   begin
      Buffer := Data (Data'First .. Data'First + Buffer'Length - 1);
   end Write_Data;

   procedure Set_Value is new RFLX.TLV.Message.Set_Value (Write_Data, Valid_Data_Length);
begin
   [...]
   Data := (1, 2, 3, 4, others => 0);
   Set_Value (Context);

In simple cases it should be as simple as:

   RFLX.TLV.Message.Set_Value (Context, (1, 2, 3, 4));