#1950: utils: add (footprinting) serializer to circular buffer

src/vt/utils/container/circular_buffer.h

@@ -105,6 +105,13 @@ struct CircularBuffer {
   bool full() const { return numFree() == 0; }
   int len() const { return size - 1 - numFree(); }
 
+  template <typename SerializerT>
+  void serialize(SerializerT& s) {
+    s | head_;
+    s | tail_;
+    s | elms_; // this is inefficient, but it's use is footprinting
+  }
+
 private:
   int head_ = 0;
   int tail_ = 0;