Add function to access element by index.

src/vec.c

@@ -103,3 +103,12 @@ int vec_foreach(vec* v, void* data, compare_func compare, foreach_func foreach,
   }
   return return_code;
 }
+
+void* vec_index(vec* v, int index){
+  void* to_return = NULL;
+  if(index < v->capacity && index >= 0) {
+    void** data_array = v->data;
+    to_return = data_array[index];
+  }
+  return to_return;
+}