Fix Word8Buffer.contents to return an object with ByteVector representation

[Skyb0rg007]

Issue

- val buf = Word8Buffer.new 10;
- Word8Buffer.add1 (buf, 0wx41);
- Byte.bytesToString (Word8Buffer.contents buf);
unexpected exception (bug?) in SML/NJ: Representation [Representation]
  raised at: Basis/Implementation/Unsafe/target64-object.sml:139.19-139.33
             ../compiler/TopLevel/print/ppobj.sml:377.24
             ../compiler/TopLevel/print/ppobj.sml:377.24
             ../compiler/TopLevel/interact/evalloop.sml:45.54

- String.map Fn.id (Byte.bytesToString (Word8Buffer.contents buf));
val it = "A" : string

Description

This change fixes the issue by ensuring the Word8Buffer.contents function returns a value with the ByteVector object representation (from create_s), instead of the ByteArray representation (from create_b).

How Has This Been Tested?

Not at all. Please test my code as I am unable compile SML/NJ myself due to an issue with the build system.