Add free conversion from Bytes back to Arc if possible

[p-avital]

The conversion is valid and free if:

• The Bytes was constructed from an Arc (ensured by the vtable comparison)
• The Arc was constructed in the same binary (ensured by the vtable comparison)
• The Bytes' wasn't shrunk (ensured by checking the start pointer and length)

Handling failure is left explicit to avoid hiding the costly version of the operation (copy into a new Arc).