Renamed len() to capacity() in EntityMap, VertexMap, and EdgeMap

len() is generally expected to describe the number of elements in a collection,
but in this case elements beyond len() are accessible without restriction.
capacity() is a better name because it describes data storage size.

src/maps.rs

@@ -13,9 +13,9 @@ 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()
     }
 
     pub fn sync<G: GraphTopology<Vertex = V>>(&mut self, graph: &G) {
@@ -48,9 +48,9 @@ impl<E: Copy, T: Clone> EdgeMap<E, 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()
     }
 
     pub fn sync<G: GraphTopology<Edge = E>>(&mut self, graph: &G) {
@@ -87,7 +87,7 @@ impl<E: Copy, T: Clone> EntityMap<E, T> {
         }
     }
 
-    pub fn len(&self) -> usize {
+    pub fn capacity(&self) -> usize {
         self.data.len()
     }