switch to the rctree crate
We currently have our own implementation of a Tree based on Rc nodes.
https://crates.io/crates/rctree provides the same functionality as a crate and it would be great to switch to it instead of carrying our own implementation.
The https://gitlab.gnome.org/pborelli/librsvg/commits/rctree branch contains a stab at this, but I am currently stuck at the last compile error: converting the CascadesValues type.