ChaiScript 6 always used a shared_ptr to wrap C++ objects created in
ChaiScript. Newer versions avoid this by only wrapping objects that cannot
be copied. This has the side effect that ChaiScript depends on correctly
implemented C++ copy and move constructors in unexpected locations.
I don't think the reduced overhead (by avoiding the shared_ptr) justifies
the new behavior in every case.
Therefore, I suggest we temporarily only perform this optimization if the class
is trivially destructible, until unexpected copies and moves in ChaiScript
are fixed (if this is even possible), or there is a sanitizer/compiler warning
that can detect these cases.
Before the change the following code will call the ctor once, move once,
and the dtor twice:
  auto obj = MoveableObject();
After this change it will only call the ctor once and the dtor once.