Enable savvy-cli to handle duplicated definitions

[yutannihilation]

Close #333

Users might use #[cfg()] to switch implementations on the same function. However, from the viewpoint of savvy-cli, who cannot evaluate #[cfg()], these are just duplicated definitions. This pull request enables the cli to ignore the duplication to generate the proper C & R bindings.

[yutannihilation]

The last failure was due to missing #[cfg()] on the *__ffi() version of function. Fixed now.

    #[cfg(not(target_os = "windows"))]
    #[allow(non_snake_case)]
    unsafe fn savvy_StructWithConfig_new_inner(
        self__: savvy::ffi::SEXP,
        x: savvy::ffi::SEXP,
    ) -> savvy::Result<StructWithConfig> {

        ...

    }
    #[allow(clippy::missing_safety_doc)]
    #[no_mangle]
    pub unsafe extern "C" fn savvy_StructWithConfig_new__ffi(
        self__: savvy::ffi::SEXP,
        x: savvy::ffi::SEXP,
    ) -> savvy::ffi::SEXP {
        match savvy_StructWithConfig_new_inner(self__, x) {

        ...

    }
    #[allow(clippy::missing_safety_doc)]
    #[no_mangle]
    pub unsafe extern "C" fn savvy_StructWithConfig_new__ffi(
        self__: savvy::ffi::SEXP,
        x: savvy::ffi::SEXP,
    ) -> savvy::ffi::SEXP {
        match savvy_StructWithConfig_new_inner(self__, x) {

        ...

        }
    }

[eitsupi]

#(#attrs)* seems needed here too.

            _ => parse_quote!(
                #[allow(clippy::missing_safety_doc)]
                #[no_mangle]
                pub unsafe extern "C" fn #fn_name_ffi( #(#args_pat: #args_ty),* ) -> savvy::ffi::SEXP {
                    match #fn_name_inner(#(#args_pat),*) {
                        Ok(#ok_lhs) => #ok_rhs,
                        Err(e) => savvy::handle_error(e),
                    }
                }
            ),

[yutannihilation]

Ouch. Thanks for catching!