clbind: disambiguate apply_and_return on std::unique_ptr

[paulapatience]

Previously, returning a std::unique_ptr from a clbound function would complain about ambiguous partial specializations of apply_and_return, the other one being include/clasp/clbind/apply.h:51.

Things I do not understand:

Why did std::move cause problems? Namely, the following:

../include/clasp/clbind/apply.h:118:98: error: non-const lvalue reference to type 'unique_ptr<...>' cannot bind to a temporary of type 'unique_ptr<...>'
  118 |     core::T_sp rv = translate::to_object<std::unique_ptr<RT>, translate::adopt_pointer>::convert(std::move(retval));
      |                                                                                                  ^~~~~~~~~~~~~~~~~

Also, what to do about the write_multiple_values call? In some other functions in apply.h, it is commented out and supplemented with the same printf I added — that is where I took the printf from. Is there really something we have to do there, or can we just remove it?

Finally, there might be other functions in apply.h that need to be disambiguated, likely the ones specialized on gctools::smart_ptr and gctools::multiple_values, but I do not understand enough of what is going on to be sure (I tried specializing them all, but that caused duplicate definitions, e.g., at lines 68 and 88).