Add Packed bytes for working with

runtime/adainclude/system-unsigned_types.ads

@@ -0,0 +1,13 @@
+package System.Unsigned_Types with
+   Pure is
+
+   type Packed_Byte is mod 2 ** 8
+      with Universal_Aliasing;
+   for Packed_Byte'Size use 8;
+
+   type Packed_Bytes1 is array (Natural range <>) of aliased Packed_Byte
+      with Suppress_Initialization;
+   for Packed_Bytes1'Alignment use 1;
+   for Packed_Bytes1'Component_Size use Packed_Byte'Size;
+
+end System.Unsigned_Types;