Move pagemap to using InvalidPointer.

Replace a constant `uintptr_t` value of 1 that was repeatedly cast to a
pointer with an `InvalidPointer<1>`.

src/ds/dllist.h

@@ -10,7 +10,7 @@ namespace snmalloc
    * Invalid pointer class.  This is similar to `std::nullptr_t`, but allows
    * other values.
    */
-  template<uintptr_t Sentinel>
+  template<address_t Sentinel>
   struct InvalidPointer
   {
     /**
@@ -40,10 +40,17 @@ namespace snmalloc
      * systems the sentinel should be a value in unmapped memory.
      */
     template<typename T>
-    operator T*()
+    operator T*() const
     {
       return reinterpret_cast<T*>(Sentinel);
     }
+    /**
+     * Implicit conversion to an address, returns the sentinel value.
+     */
+    operator address_t() const
+    {
+      return Sentinel;
+    }
   };
 
   template<class T, class Terminator = std::nullptr_t>