We need this for a couple of special cases that are *not*
implementations of node methods and yet want to use the impl structures.
For example, rsvg_handle_get_dimensions_sub() wants to peek at the
toplevel svg->vbox.

We add a Node.get_c_impl() method, and a NodeTrait.get_c_impl().  The
CNode actually implements this last one by returning its stored
c_node_impl.  Nodes implemented fully in Rust will return just a null
pointer.

We can hopefully remove that method once everything is implemented in Rust.

* Move rsvg-structure.[ch] over to rsvg_rust_cnode_new()

The new rsvg_foo_node_new() functions use rsvg_rust_cnode_new()
internally instead of the old _rsvg_node_init().  Also, the RsvgFooNode
structs don't have a "super" field anymore, since RsvgNode is now an
opaque struct from the Rust code.

We re-do the virtual methods for all of rsvg-structure.c.  Other files
will follow.

* Move rsvg-shapes.[ch] over to rsvg_rust_cnode_new().