[Python] Support recursive shapes that are boxed in Rust

[crisidev]

Recursive shapes are Boxed in Rust and pyo3 does not implement FromPyObj and IntoPy for Box<T>.

This can be solved by implementing these 2 traits for Box<T> and Option<Box<T> where T is not any concrete type.

More info PyO3/pyo3#2090

[crisidev]

For anything that is part of a pyo3 function definition like

#[pymethods]
impl MyStruct {
    pub fn newpy(
        arg1: Box<OtherStruct>, 
        arg2: Option<Box<OtherStruct>>, 
    ) -> Self {
        ...
    }
}

we can use the macro #[pyo3(from_py_with = "extractor_func")]

For example:

fn make_box<T>(src: &pyo3::PyAny) -> pyo3::PyResult<Box<T>>
where
    T: for<'a> pyo3::FromPyObject<'a>
{
    src.extract().map(Box::new)
}
fn make_optional_box<T>(src: &pyo3::PyAny) -> pyo3::PyResult<Option<Box<T>>>
where
    T: for<'a> pyo3::FromPyObject<'a> 
{
    ...
}

The implementation will become:

#[pymethods]
impl MyStruct {
    pub fn newpy(
        #[pyo3(from_py_with = "make_box")]
        arg1: Box<OtherStruct>, 
        #[pyo3(from_py_with = "make_optional_box")]
        arg2: Option<Box<OtherStruct>>, 
    ) -> Self {
        ...
    }
}

But I think this is not enough and it would be better to just generate the implementation of FromPyObj and IntoPy traits as from_py_with seems to be useable only in function signature and not in structure definitions.