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>

src/mem/pagemap.h

@@ -78,7 +78,7 @@ namespace snmalloc
       (INDEX_LEVELS * BITS_PER_INDEX_LEVEL) + BITS_FOR_LEAF + GRANULARITY_BITS;
 
     // Value used to represent when a node is being added too
-    static constexpr address_t LOCKED_ENTRY = 1;
+    static constexpr InvalidPointer<1> LOCKED_ENTRY;
 
     struct Leaf
     {
@@ -112,16 +112,14 @@ namespace snmalloc
       // to see that correctly.
       PagemapEntry* value = e->load(std::memory_order_relaxed);
 
-      if (address_cast(value) <= LOCKED_ENTRY)
+      if ((value == nullptr) || (value == LOCKED_ENTRY))
       {
         if constexpr (create_addr)
         {
           value = nullptr;
 
           if (e->compare_exchange_strong(
-                value,
-                pointer_cast<PagemapEntry>(LOCKED_ENTRY),
-                std::memory_order_relaxed))
+                value, LOCKED_ENTRY, std::memory_order_relaxed))
           {
             auto& v = default_memory_provider;
             value = v.alloc_chunk<PagemapEntry, OS_PAGE_SIZE>();