Replace VertexMap/EdgeMap.len with capacity

Rename EdgeMap.len to capacity.
Add VertexMap.capacity and EdgeMap.capacity.
Deprecate VertexMap.len and EdgeMap.len.

src/maps.rs

@@ -13,11 +13,17 @@ impl<V: Copy, T: Clone> VertexMap<V, T> {
         }
     }
 
-    // Reads beyond len() are valid and return the default value.
-    pub fn len(&self) -> usize {
-        self.inner.len()
+    // Reads beyond 'capacity' are valid and return the default value.
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
     }
 
+    #[deprecated(since = "0.2.2", note = "use 'capacity' instead")]
+    pub fn len(&self) -> usize {
+        self.capacity()
+    }
+
+
     pub fn sync<G: GraphTopology<Vertex = V>>(&mut self, graph: &G) {
         self.inner.resize(graph.vertex_capacity());
     }
@@ -48,9 +54,14 @@ impl<E: Copy, T: Clone> EdgeMap<E, T> {
         }
     }
 
-    // Reads beyond len() are valid and return the default value.
+    // Reads beyond 'capacity' are valid and return the default value.
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
+    }
+
+    #[deprecated(since = "0.2.2", note = "use 'capacity' instead")]
     pub fn len(&self) -> usize {
-        self.inner.len()
+        self.capacity()
     }
 
     pub fn sync<G: GraphTopology<Edge = E>>(&mut self, graph: &G) {
@@ -87,7 +98,7 @@ impl<E: Copy, T: Clone> EntityMap<E, T> {
         }
     }
 
-    pub fn len(&self) -> usize {
+    pub fn capacity(&self) -> usize {
         self.data.len()
     }