Tidy TODOs from Free List

* Add extra key to freelist.  This follows the encoding Cedric suggested
  for a signature of two things. Free list key now has a pair of keys
  for encoding previous pointer. This makes it harder to extract the
  underlying keys out of the multiplication.

* Apply SFINAE to the extract_segment.

src/mem/entropy.h

@@ -31,11 +31,13 @@ namespace snmalloc
 
   struct FreeListKey
   {
-    address_t key;
+    address_t key1;
+    address_t key2;
     address_t key_next;
 
-    constexpr FreeListKey(uint64_t key, uint64_t key_next)
-    : key(static_cast<address_t>(key)),
+    constexpr FreeListKey(uint64_t key1, uint64_t key2, uint64_t key_next)
+    : key1(static_cast<address_t>(key1)),
+      key2(static_cast<address_t>(key2)),
       key_next(static_cast<address_t>(key_next))
     {}
   };
@@ -47,7 +49,7 @@ namespace snmalloc
     uint64_t local_counter{0};
     uint64_t fresh_bits{0};
     uint64_t count{0};
-    FreeListKey key{0, 0};
+    FreeListKey key{0, 0, 0};
 
   public:
     constexpr LocalEntropy() = default;
@@ -59,12 +61,14 @@ namespace snmalloc
       local_counter = get_entropy64<PAL>();
       if constexpr (bits::BITS == 64)
       {
-        key.key = get_next();
+        key.key1 = get_next();
+        key.key2 = get_next();
         key.key_next = get_next();
       }
       else
       {
-        key.key = get_next() & 0xffff'ffff;
+        key.key1 = get_next() & 0xffff'ffff;
+        key.key2 = get_next() & 0xffff'ffff;
         key.key_next = get_next() & 0xffff'ffff;
       }
       bit_source = get_next();

src/mem/freelist.h

@@ -43,20 +43,13 @@ namespace snmalloc
 {
   /**
    * This function is used to sign back pointers in the free list.
-   *
-   * TODO - Needs review.  Should we shift low bits out as they help
-   * guess the key.
-   *
-   * TODO - We now have space in the FreeListBuilder for a fresh key for each
-   * list.
    */
   inline static uintptr_t
   signed_prev(address_t curr, address_t next, const FreeListKey& key)
   {
     auto c = curr;
     auto n = next;
-    auto k = key.key;
-    return (c + k) * (n - k);
+    return (c + key.key1) * (n + key.key2);
   }
 
   /**
@@ -65,7 +58,7 @@ namespace snmalloc
    * back pointer in a doubly linked list, however, it is encoded
    * to prevent corruption.
    *
-   * TODO: Consider put prev_encoded at the end of the object, would
+   * TODO: Consider putting prev_encoded at the end of the object, would
    * require size to be threaded through, but would provide more OOB
    * detection.
    */
@@ -443,11 +436,14 @@ namespace snmalloc
       }
     }
 
-    std::pair<CapPtr<FreeObject, CBAlloc>, CapPtr<FreeObject, CBAlloc>>
+    template<bool RANDOM_ = RANDOM>
+    std::enable_if_t<
+      !RANDOM_,
+      std::pair<CapPtr<FreeObject, CBAlloc>, CapPtr<FreeObject, CBAlloc>>>
     extract_segment(const FreeListKey& key)
     {
+      static_assert(RANDOM_ == RANDOM, "Don't set SFINAE parameter!");
       SNMALLOC_ASSERT(!empty());
-      SNMALLOC_ASSERT(!RANDOM); // TODO: Turn this into a static failure.
 
       auto first = read_head(0, key);
       // end[0] is pointing to the first field in the object,

src/mem/remoteallocator.h

@@ -27,7 +27,7 @@ namespace snmalloc
   /**
    * Global key for all remote lists.
    */
-  inline static FreeListKey key_global(0xdeadbeef, 0xdeadbeef);
+  inline static FreeListKey key_global(0xdeadbeef, 0xbeefdead, 0xdeadbeef);
 
   struct alignas(REMOTE_MIN_ALIGN) RemoteAllocator
   {