Resolve issues with Str introduced when it became ownable

Resolve issues with Str introduced when it became ownable

When converting from a Str to an &str, we previously took ownership
of the Str, copied the pointer to its characters to an &str and
returned the &str. This made the returned bytes always unreadable.

Further, there is now no reason to use Vec_u8Z for owned Strs, so
we swap for Str as well.