range_map: Add a method to find which range a single value falls into.

src/base/range_map.hh

@@ -69,6 +69,13 @@ class range_map
         return tree.end();
     }
 
+    template <class U>
+    const iterator
+    find(const U &r)
+    {
+        return find(RangeSize(r, 1));
+    }
+
     template <class U>
     bool
     intersect(const Range<U> &r)