Feature Request: Add support for nested Vector types

[IBims1NicerTobi]

It would be great if there was some way to pass a Vec<Vec> from Rust to C++. This is currently not possible without applying some hack with a struct and a Vec inside that struct which is very annoying and leads to extra boilerplate code on the C++ side.
MWE:

#[cxx::bridge(namespace="test")]
mod ffi {
    extern "Rust" {
        fn test() -> Vec<Vec<f32>>;
    }
}

[mbrobbel]

#671