Allow subclassed P5 objects to access their own data (fixes Moose test).

A P5 object passed back from P6 is again wrapped to allow subclassing and
dispatching method calls from P5 to P6. But the wrapped object was a
completely new scalar and the original object was hidden from P5 methods.

Re-bless the original P5 object instead. This allows its methods to access
the object's data while still allowing the P6 class to overwrite methods.
Re-bless P5 object only once to Perl6::Object

No need to bless it every time we pass it back to P5. Just do it once in the
constructor of the subclass like we would do it in plain normal P5 code.