This works by transferring ownership of the underlying C++ memory
from the target language proxy class to an instance of the unique_ptr
which is passed to the wrapped function via std::move.
The proxy class has a new swigRelease() method which sets the
underlying C++ pointer for the proxy class to null, so working
in much the same way as std::unique_ptr::release(). Any attempt at
using the proxy class will be the same as if the delete() function
has been called on the proxy class. That is, using a C++ null pointer,
when a non-null pointer is usually expected.
This commit relies on the previous commit that uses std::move
on the temporary variable used for the wrapped function's input parameter
as std::unique_ptr is not copyable, it has move-only semantics.
