Export pyobject_* macros

[termoshtt]

Hi, thanks for keeping this project :)

I am trying to implement the backend of rust-numpy with PyO3. I am implementing traits in PyO3, e.g. ToPyObject for the struct PyArray(PyObject), which wraps the PyArrayObject in NumPy C API.
The implementations of these traits for most primitive types, e.g. PyList, are automatically derived using pyobject_nativetype! and other pyobject_* macros. I would like to use these macros also for PyArray. Could you expose these macros using #[macro_export]?
If you have a plan to replace them by custom derives, I willing to help it.