Make CSerAction's ForRead() constexpr

The CSerAction's ForRead() method does not depend on any runtime
data, so guarantee that requests to it can be optimized out by
making it constexpr.

Suggested by Cory Fields.

src/serialize.h

@@ -773,11 +773,11 @@ void Unserialize(Stream& is, std::set<K, Pred, A>& m)
  */
 struct CSerActionSerialize
 {
-    bool ForRead() const { return false; }
+    constexpr bool ForRead() const { return false; }
 };
 struct CSerActionUnserialize
 {
-    bool ForRead() const { return true; }
+    constexpr bool ForRead() const { return true; }
 };
 
 template<typename Stream, typename T>