Add way to check if a refcounted object has other references.

c++/src/kj/refcount-test.c++

@@ -34,13 +34,18 @@ struct SetTrueInDestructor: public Refcounted {
 TEST(Refcount, Basic) {
   bool b = false;
   Own<SetTrueInDestructor> ref1 = kj::refcounted<SetTrueInDestructor>(&b);
+  EXPECT_FALSE(ref1->isShared());
   Own<SetTrueInDestructor> ref2 = kj::addRef(*ref1);
+  EXPECT_TRUE(ref1->isShared());
   Own<SetTrueInDestructor> ref3 = kj::addRef(*ref2);
+  EXPECT_TRUE(ref1->isShared());
 
   EXPECT_FALSE(b);
   ref1 = Own<SetTrueInDestructor>();
+  EXPECT_TRUE(ref2->isShared());
   EXPECT_FALSE(b);
   ref3 = Own<SetTrueInDestructor>();
+  EXPECT_FALSE(ref2->isShared());
   EXPECT_FALSE(b);
   ref2 = Own<SetTrueInDestructor>();
   EXPECT_TRUE(b);

c++/src/kj/refcount.h

@@ -59,6 +59,10 @@ class Refcounted: private Disposer {
 public:
   virtual ~Refcounted() noexcept(false);
 
+  inline bool isShared() const { return refcount > 1; }
+  // Check if there are multiple references to this object. This is sometimes useful for deciding
+  // whether it's safe to modify the object vs. make a copy.
+
 private:
   mutable uint refcount = 0;
   // "mutable" because disposeImpl() is const.  Bleh.