Add some methods to the Point record

This code works, because `declare_interface` auto-inserts the `pub use geometry::Point` statement.
As a Rust dev this _reads_ weird, because conceptually `Point` is in a
submodule that is no where explicitly `use`d.
This might be a bit too much magic.

examples/geometry/src/lib.rs

@@ -48,3 +48,10 @@ mod geometry {
         })
     }
 }
+
+impl Point {
+    fn move_to(&mut self, x: f64, y: f64) {
+        self.coord_x += x;
+        self.coord_y += y;
+    }
+}