Remove impl RsvgNode
More preliminary refactoring for the switch to rctree, but I think it makes things cleaner also on its own.
It seems to me that having an impl RsvgNode mixes the tree structure with the manipulation of each node: I like making it explicit that we borrow the data from node and use it.
I will rebase my rctree branch on top of this... on that one, I am stuck on one last error related to CascadedValues. I think I will open an issue so that it is easier to discuss.