LibJS: Rename Allocator => CellAllocator

Now that we have a BlockAllocator as well, it seems appropriate to name
the allocator-that-allocates-cells something more specific to match.

Userland/Libraries/LibJS/Heap/CellAllocator.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/IntrusiveList.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/Vector.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Heap/HeapBlock.h>
+
+namespace JS {
+
+class CellAllocator {
+public:
+    explicit CellAllocator(size_t cell_size);
+    ~CellAllocator();
+
+    size_t cell_size() const { return m_cell_size; }
+
+    Cell* allocate_cell(Heap&);
+
+    template<typename Callback>
+    IterationDecision for_each_block(Callback callback)
+    {
+        for (auto& block : m_full_blocks) {
+            if (callback(block) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        for (auto& block : m_usable_blocks) {
+            if (callback(block) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    }
+
+    void block_did_become_empty(Badge<Heap>, HeapBlock&);
+    void block_did_become_usable(Badge<Heap>, HeapBlock&);
+
+private:
+    const size_t m_cell_size;
+
+    typedef IntrusiveList<HeapBlock, RawPtr<HeapBlock>, &HeapBlock::m_list_node> BlockList;
+    BlockList m_full_blocks;
+    BlockList m_usable_blocks;
+};
+
+}