Support `take_ownership` for custom type casters given a pointer

This changes the pointer `cast()` in `PYBIND11_TYPE_CASTER` to recognize
the `take_ownership` policy: if casting a pointer with take-ownership,
the `cast()` now recalls `cast()` with a dereferenced rvalue (rather
than the previous code, which was always calling it with a const lvalue
reference), and deletes the pointer after the chained `cast()` is
complete.

This makes code like:

    m.def("f", []() { return new std::vector<int>(100, 1); },
        py::return_value_policy::take_ownership);

do the expected thing by taking over ownership of the returned pointer
(which is deleted once the chained cast completes).