Add an insert function to our list utility class.

src/list.h

Index: src/list.h
===================================================================
--- src/list.h	(revision 1407)
+++ src/list.h	(working copy)
@@ -79,6 +79,11 @@
   // until the next change is made to this list.
   Vector<T> AddBlock(const T& value, int count);
 
+  // Inserts a copy of the given  element at index i in the list.  All
+  // elements formerly  at or above i  are moved up and  the length of
+  // the list increases by one.
+  T& Insert(int i, const T& element);
+
   // Removes the i'th element without deleting it even if T is a
   // pointer type; moves all elements above i "down". Returns the
   // removed element.