Add Last(). NFC.

Counterpart of First().  standard library calls this "back()".

src/dsc.h

@@ -358,7 +358,7 @@ public:
         if(IsEmpty()) {
             return 0;
         } else {
-            return elem[n - 1].h.v;
+            return Last()->h.v;
         }
     }
 
@@ -455,6 +455,9 @@ public:
     T *First() {
         return (IsEmpty()) ? NULL : &(elem[0]);
     }
+    T *Last() {
+        return (IsEmpty()) ? NULL : &(elem[n-1]);
+    }
     T *NextAfter(T *prev) {
         if(IsEmpty() || !prev) return NULL;
         if(prev - elem == (n - 1)) return NULL;