Change to inserting an external object into the wrapper object's
prototype chain, instead of injecting an alternate `this` object
in the constructor callback adapter. The latter approach didn't
work in certain scenarios because the JSRT runtime still returned
the original `this` object.

And with this change, the setting and checking of the extension
flag, which distinguished wrapper objects from external objects,
can be removed. I was never happy with that abuse of the extension
flag even when I coded it.

Also removing the CallbackInfo.returnValue field which is not
used anymore (since we switched to direct return values).