ffi: define compat for Py_NewRef and Py_XNewRef

[davidhewitt]

This improves the FFI definitions Py_NewRef and Py_XNewRef in the following ways:

• Adds #[doc(cfg(Py_3_10))] to the original definitions (which are either an extern symbol or an inline function depending on abi3 setting, and so would a cfg hint in the doc saying they were only available in one of these)
• Adds Py_NewRef and Py_XNewRef to pyo3_ffi::compat, so we can use them on all versions internally
• Removes the private implementation details _Py_NewRef and _Py_XNewRef

To make the doc-cfgs on pyo3_ffi::compat work properly, I realised that I needed to use functions rather than re-exports when running the docrs build. I decided to use a macro to make this implementation easier. This way the compat functions are identical regardless of what Python version we build with. (cc @mejrs)

This is what I saw before (clicking on these re-exports then shows a version cfg):

After:

[mejrs]

Thanks!

I think we should add a test to ensure glob re-exports do not trigger import ambiguity:

mod test_export{
     pub use pyo3_ffi::compat::*;
     pub use pyo3_ffi::*;
}

[davidhewitt]

Great idea. It turned out to be a little bit harder than just that pair of use statements, because to trigger the ambiguity error I actually needed to use an ambiguous name. What I ended up doing was adding a test to the code generated from the compat_function! macro so that it automatically set up a check. This should automatically ensure new symbols in ffi::compat are never ambiguous 😄