Add validate to freelist::Builder

The free list builder in a checked build will only validate entries when
they are removed.  This commit adds a validate method, so they can be
checked during teardown.  This means that programs that leak memory
will still fail if the free list has become corrupt.

src/mem/freelist.h

@@ -116,6 +116,13 @@ namespace snmalloc
       template<SNMALLOC_CONCEPT(capptr::ConceptBound) BQueue>
       class T
       {
+        template<
+          bool,
+          bool,
+          SNMALLOC_CONCEPT(capptr::ConceptBound),
+          SNMALLOC_CONCEPT(capptr::ConceptBound)>
+        friend class Builder;
+
         friend class Object;
 
         union
@@ -699,6 +706,40 @@ namespace snmalloc
         init();
         return {first, last};
       }
+
+      template<typename Domesticator>
+      SNMALLOC_FAST_PATH void
+      validate(const FreeListKey& key, Domesticator domesticate)
+      {
+#ifdef SNMALLOC_CHECK_CLIENT
+        for (uint32_t i = 0; i < LENGTH; i++)
+        {
+          if (&head[i] == end[i])
+          {
+            SNMALLOC_ASSERT(length[i] == 0);
+            continue;
+          }
+
+          size_t count = 1;
+          auto curr = read_head(i, key);
+          auto prev = get_fake_signed_prev(i, key);
+          while (true)
+          {
+            curr->check_prev(prev);
+            if (address_cast(&(curr->next_object)) == address_cast(end[i]))
+              break;
+            count++;
+            auto next = curr->read_next(key, domesticate);
+            prev = signed_prev(address_cast(curr), address_cast(next), key);
+            curr = next;
+          }
+          SNMALLOC_ASSERT(count == length[i]);
+        }
+#else
+        UNUSED(key);
+        UNUSED(domesticate);
+#endif
+      }
     };
   } // namespace freelist
 } // namespace snmalloc