Add API for adding surfaces to Shape

The surfaces aren't actually stored yet, as that is not necessary right
now. Going through this API paves the way for future additions though.

src/kernel/algorithms/approximation.rs

@@ -266,8 +266,9 @@ mod tests {
             edges: vec![ab, bc, cd, da],
         });
 
+        let surface = shape.surfaces().add(Surface::x_y_plane());
         let face = Face::Face {
-            surface: Surface::x_y_plane(),
+            surface,
             cycles: vec![abcd],
         };
 

src/kernel/algorithms/sweep.rs

@@ -129,10 +129,14 @@ mod tests {
                 edges: vec![ab, bc, ca],
             });
 
+            let surface =
+                shape
+                    .surfaces()
+                    .add(Surface::Swept(Swept::plane_from_points(
+                        [a, b, c].map(|vertex| vertex.point()),
+                    )));
             let abc = Face::Face {
-                surface: Surface::Swept(Swept::plane_from_points(
-                    [a, b, c].map(|vertex| vertex.point()),
-                )),
+                surface,
                 cycles: vec![cycles],
             };
 

src/kernel/algorithms/transform.rs

@@ -65,9 +65,11 @@ pub fn transform_face(
                 cycles_trans.push(shape.cycles().add(Cycle { edges }));
             }
 
+            let surface = shape.surfaces().add(surface.transform(transform));
+
             Face::Face {
                 cycles: cycles_trans,
-                surface: surface.transform(transform),
+                surface,
             }
         }
         Face::Triangles(mut triangles) => {

src/kernel/shape/mod.rs

@@ -2,6 +2,7 @@ pub mod cycles;
 pub mod edges;
 pub mod faces;
 pub mod handle;
+pub mod surfaces;
 pub mod vertices;
 
 use crate::math::Scalar;
@@ -10,7 +11,7 @@ use super::topology::{edges::Cycle, faces::Face, vertices::Vertex};
 
 use self::{
     cycles::Cycles, edges::Edges, faces::Faces, handle::Storage,
-    vertices::Vertices,
+    surfaces::Surfaces, vertices::Vertices,
 };
 
 /// The boundary representation of a shape
@@ -56,6 +57,11 @@ impl Shape {
         self
     }
 
+    /// Access the shape's surfaces
+    pub fn surfaces(&mut self) -> Surfaces {
+        Surfaces
+    }
+
     /// Access the shape's vertices
     pub fn vertices(&mut self) -> Vertices {
         Vertices {

src/kernel/shape/surfaces.rs

@@ -0,0 +1,13 @@
+use crate::kernel::geometry::Surface;
+
+use super::handle::{Handle, Storage};
+
+/// API to access the surfaces of a shape
+pub struct Surfaces;
+
+impl Surfaces {
+    /// Add a surface to the shape
+    pub fn add(&mut self, surface: Surface) -> Handle<Surface> {
+        Storage::new(surface).handle()
+    }
+}

src/kernel/shapes/circle.rs

@@ -26,10 +26,8 @@ impl ToShape for fj::Circle {
         shape.cycles().add(Cycle { edges: vec![edge] });
 
         let cycles = shape.cycles().all().collect();
-        shape.faces().add(Face::Face {
-            cycles,
-            surface: Surface::x_y_plane(),
-        });
+        let surface = shape.surfaces().add(Surface::x_y_plane());
+        shape.faces().add(Face::Face { cycles, surface });
 
         shape
     }

src/kernel/shapes/sketch.rs

@@ -47,9 +47,10 @@ impl ToShape for fj::Sketch {
             shape.cycles().add(Cycle { edges });
         };
 
+        let surface = shape.surfaces().add(Surface::x_y_plane());
         let face = Face::Face {
             cycles: shape.cycles().all().collect(),
-            surface: Surface::x_y_plane(),
+            surface,
         };
         shape.faces().add(face);
 

src/kernel/topology/faces.rs

@@ -26,7 +26,7 @@ pub enum Face {
     /// surface.
     Face {
         /// The surface that defines this face
-        surface: Surface,
+        surface: Handle<Surface>,
 
         /// The cycles that bound the face
         ///