LibGC+Everywhere: Factor out a LibGC from LibJS

Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:

 * JS::NonnullGCPtr -> GC::Ref
 * JS::GCPtr -> GC::Ptr
 * JS::HeapFunction -> GC::Function
 * JS::CellImpl -> GC::Cell
 * JS::Handle -> GC::Root

Libraries/LibGC/CellAllocator.h

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2020-2023, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/IntrusiveList.h>
+#include <AK/NeverDestroyed.h>
+#include <AK/NonnullOwnPtr.h>
+#include <LibGC/BlockAllocator.h>
+#include <LibGC/Forward.h>
+#include <LibGC/HeapBlock.h>
+
+#define GC_DECLARE_ALLOCATOR(ClassName) \
+    static GC::TypeIsolatingCellAllocator<ClassName> cell_allocator
+
+#define GC_DEFINE_ALLOCATOR(ClassName) \
+    GC::TypeIsolatingCellAllocator<ClassName> ClassName::cell_allocator { #ClassName }
+
+namespace GC {
+
+class CellAllocator {
+public:
+    CellAllocator(size_t cell_size, char const* class_name = nullptr);
+    ~CellAllocator() = default;
+
+    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&);
+
+    IntrusiveListNode<CellAllocator> m_list_node;
+    using List = IntrusiveList<&CellAllocator::m_list_node>;
+
+    BlockAllocator& block_allocator() { return m_block_allocator; }
+    FlatPtr min_block_address() const { return m_min_block_address; }
+    FlatPtr max_block_address() const { return m_max_block_address; }
+
+private:
+    char const* const m_class_name { nullptr };
+    size_t const m_cell_size;
+
+    BlockAllocator m_block_allocator;
+
+    using BlockList = IntrusiveList<&HeapBlock::m_list_node>;
+    BlockList m_full_blocks;
+    BlockList m_usable_blocks;
+    FlatPtr m_min_block_address { explode_byte(0xff) };
+    FlatPtr m_max_block_address { 0 };
+};
+
+template<typename T>
+class TypeIsolatingCellAllocator {
+public:
+    using CellType = T;
+
+    TypeIsolatingCellAllocator(char const* class_name)
+        : allocator(sizeof(T), class_name)
+    {
+    }
+
+    NeverDestroyed<CellAllocator> allocator;
+};
+
+}