This patch adds an explicit 'destruct' function argument to registry::push_back().

Motivation:
See the 'opaque_ref' test in commit 834f0d1. This creates a wrapper for a C++ type where only the declaration is known. This may happen if you want to wrap functions/methods which take a const ref to a certain type as an argument, but instances of that type are produced elsewhere (perhaps a different Python module).
As it is now the 'opaque_ref' test fails because the associated Python module references ~Opaque() - which is undefined. This is also counterintuitive, since Opaque is actually not constructed anywhere!
The culprit is rvalue_from_python_data<T>::~rvalue_from_python_data() which unconditionally references the dtor of T. If we replace this dtor call with an explicit destructor function we only need a dtor reference if we're actually creating an instance.

Source compatibility?:
Adding a parameter to registry::push_back() may break existing sources. It's not clear to me whether it's supposed to be a "stable" API or whether breakage is acceptable. If it's the former I'd look into creating a source-compatible variant.