pyfunction: allow wrap_pyfunction to work on imports (even cross-crate)

[davidhewitt]

I'm really pumped about this! I had an idea earlier today how to make wrap_pyfunction! work on a #[pyfunction] imported from another module.

How it works: #[pyfunction] fn foo now generates a hidden mod foo alongside the function. They're allowed to coexist, and use foo will bring both into the new scope. The hidden module contains just enough exports for wrap_pyfunction to be able to get what it needs. See pyo3-macros-backend/src/pyfunction.rs for the details.

This works really nicely - it even works across different crates! 🚀

(This trick could potentially also be used for #[pymodule] / wrap_pymodule!, though I'll save that for another day, because things may also change when we implement #[pymodule] on mod as proposed in #694 (comment).)

[adamreichold]

This will really remove a lot of boiler plate init functions from some code bases I work with and remove a cost factor from having more fine-grained Rust-side modules. Thanks!

[frameadvisors]

I am still getting this error. Is there a example/template that someone could share?