CxxFunction + rust::Closure

[smessmer]

It would be nice to have an effortless way to pass closures across ffi boundaries.

• Similar to CxxVector in Rust, there could be a CxxFunction that wraps a std::function and allows calling it.
• Similar to rust::Vec in C++, there could be a rust::Fn that wraps a Rust Box<dyn Fn> and allows calling it. Note that to be actually useful, this would have to be combined with generic support as proposed in https://github.com/dtolnay/cxx/issues/683