Add a generic remembered set class.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 11ce656a2ef79810b74369c07ef3e3d3c35f8ae2..055b176f735da231c262047456dce505edeb70b6 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -291,6 +291,7 @@ class Scavenger;
 class ScavengeJob;
 class WeakObjectRetainer;
 
+typedef void (*ObjectSlotCallback)(HeapObject** from, HeapObject* to);
 
 // A queue of objects promoted during scavenge. Each object is accompanied
 // by it's size to avoid dereferencing a map pointer for scanning.