pyfunction: use extract_argument with holder to avoid extractext

[davidhewitt]

Motivated by #2487, this is hopefully a better long-term solution than patching to avoid upsetting clippy.

This is a reworking of the pyfunction macro implementations to have some smarter internal type inference for extracting reference values. At the moment there is an ExtractExt trait and a lot of generated macro code dancing with derefs which make clippy quite unhappy.

This PR replaces ExtractExt with a PyFunctionArgument trait which avoids the need for deref juggling by taking a &mut holder argument used as temporary storage. For T: FromPyObject types the holder is just () (and should hopefully be optimised out), and for &T / &mut T for #[pyclass] types the holder is Option<PyRef<T>> and Option<PyRefMut<T>> (Option because if the wrong argument is passed from Python then the holder never gets populated).

I was careful to check both benchmarks and generated lines of code, and I got no noticeable difference to either. The net result is more friendly on type inference, leads to simpler macro code, and avoids clippy trouble.

To have a practical example of what this change results as - for a function which looks like below:

#[pyclass]
struct Foo;

#[pyfunction]
fn foo(_value: &Foo) { }

The relevant generated code has changed from:

let _tmp: <&Foo as _pyo3::derive_utils::ExtractExt<'_>>::Target =
    _pyo3::impl_::extract_argument::extract_argument(
        _pyo3::impl_::extract_argument::unwrap_required_argument(output[0usize]),
        "_value",
    )?;
#[allow(clippy::needless_option_as_deref)]
let arg0 = &*_tmp;
foo(arg0);

to:

foo(
    _pyo3::impl_::extract_argument::extract_argument(
        _pyo3::impl_::extract_argument::unwrap_required_argument(output[0usize]),
        &mut { _pyo3::impl_::extract_argument::FunctionArgumentHolder::INIT },
        "_value",
    )?
);

[davidhewitt]

For the "bad import" case described in #2487, this PR appears to also cleanly solve that:

// missing PathBuf import
use pyo3::prelude::*;

#[pyfunction]
fn foo(_path: Option<PathBuf>) { }

gives an error exactly as one would hope:

error[E0412]: cannot find type `PathBuf` in this scope
 --> src/lib.rs:6:22
  |
6 | fn foo(_path: Option<PathBuf>) { }
  |                      ^^^^^^^ not found in this scope
  |
help: consider importing this struct
  |
3 | use std::path::PathBuf;
  |

[davidhewitt]

I'm sufficiently convinced that this is better than the ExtractExt implementation that I will proceed to merge now.

[mejrs]

Thanks :)