Add support for `traverse` in the FFI interface

[cowlicks]

The Traverse struct is not Send so it can't be exposed with via UniFFI.

My plan to work around this is to use message passing to pass the traverse data into the FFI land. This will require a deamon like thing in Rust land to handle the Traverse object and send the data it produces to a client like thing in FFI land. I expect cleaning up the Traverse objects might be tricky.