Remove pub from structs and fields that should have been private

Graph fields can now be private because of the new iterators Graph::vertices()
and Graph::neighbors(). Without them there was no way to access vertices and
incidences without the public Graph fields.

src/models.rs

@@ -2,15 +2,12 @@
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct Vertex(pub usize);
 
-// TODO: Incidence value must not be public.
-// TODO: Incidence must not be public.
 #[derive(Copy, Clone, PartialEq, Eq)]
-pub struct Incidence(pub usize);
+struct Incidence(usize);
 
-// TODO: VertexIncidenceHeader and its fields must not be public, but without iterators there is currently no other way to access vertex incidences.
-pub struct VertexIncidenceHeader {
+struct VertexIncidenceHeader {
     incidence_count: usize,
-    pub first_incidence: Option<Incidence>,
+    first_incidence: Option<Incidence>,
 }
 
 pub struct VertexNeighborIterator<'a> {
@@ -28,7 +25,6 @@ impl<'a> Iterator for VertexNeighborIterator<'a> {
     }
 }
 
-// TODO: Graph fields must not be public.
 // TODO: Add functions to reserve memory for vertices and edges.
 // TODO: Graph cannot support deleting vertices because of the external indices "Vertex". They could be made stable with e.g. slotmap or generational arena.
 // TODO: Add function to delete vertices.
@@ -36,9 +32,9 @@ impl<'a> Iterator for VertexNeighborIterator<'a> {
 // TODO: Add iterator of edges.
 pub struct Graph {
     // TODO: Index 'incidence_headers' by a 'Vertex' instead of 'Vertex.0'?
-    pub incidence_headers: Vec<VertexIncidenceHeader>,
-    pub next_incidences: Vec<Option<Incidence>>,
-    pub incidence_vertices: Vec<Vertex>,
+    incidence_headers: Vec<VertexIncidenceHeader>,
+    next_incidences: Vec<Option<Incidence>>,
+    incidence_vertices: Vec<Vertex>,
 }
 
 impl Graph {