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/HeapBlock.h

@@ -0,0 +1,121 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/IntrusiveList.h>
+#include <AK/Platform.h>
+#include <AK/StringView.h>
+#include <AK/Types.h>
+#include <LibGC/Cell.h>
+#include <LibGC/Forward.h>
+#include <LibGC/Internals.h>
+
+#ifdef HAS_ADDRESS_SANITIZER
+#    include <sanitizer/asan_interface.h>
+#endif
+
+namespace GC {
+
+class HeapBlock : public HeapBlockBase {
+    AK_MAKE_NONCOPYABLE(HeapBlock);
+    AK_MAKE_NONMOVABLE(HeapBlock);
+
+public:
+    using HeapBlockBase::block_size;
+    static NonnullOwnPtr<HeapBlock> create_with_cell_size(Heap&, CellAllocator&, size_t cell_size, char const* class_name);
+
+    size_t cell_size() const { return m_cell_size; }
+    size_t cell_count() const { return (block_size - sizeof(HeapBlock)) / m_cell_size; }
+    bool is_full() const { return !has_lazy_freelist() && !m_freelist; }
+
+    ALWAYS_INLINE Cell* allocate()
+    {
+        Cell* allocated_cell = nullptr;
+        if (m_freelist) {
+            VERIFY(is_valid_cell_pointer(m_freelist));
+            allocated_cell = exchange(m_freelist, m_freelist->next);
+        } else if (has_lazy_freelist()) {
+            allocated_cell = cell(m_next_lazy_freelist_index++);
+        }
+
+        if (allocated_cell) {
+            ASAN_UNPOISON_MEMORY_REGION(allocated_cell, m_cell_size);
+        }
+        return allocated_cell;
+    }
+
+    void deallocate(Cell*);
+
+    template<typename Callback>
+    void for_each_cell(Callback callback)
+    {
+        auto end = has_lazy_freelist() ? m_next_lazy_freelist_index : cell_count();
+        for (size_t i = 0; i < end; ++i)
+            callback(cell(i));
+    }
+
+    template<Cell::State state, typename Callback>
+    void for_each_cell_in_state(Callback callback)
+    {
+        for_each_cell([&](auto* cell) {
+            if (cell->state() == state)
+                callback(cell);
+        });
+    }
+
+    static HeapBlock* from_cell(Cell const* cell)
+    {
+        return static_cast<HeapBlock*>(HeapBlockBase::from_cell(cell));
+    }
+
+    Cell* cell_from_possible_pointer(FlatPtr pointer)
+    {
+        if (pointer < reinterpret_cast<FlatPtr>(m_storage))
+            return nullptr;
+        size_t cell_index = (pointer - reinterpret_cast<FlatPtr>(m_storage)) / m_cell_size;
+        auto end = has_lazy_freelist() ? m_next_lazy_freelist_index : cell_count();
+        if (cell_index >= end)
+            return nullptr;
+        return cell(cell_index);
+    }
+
+    bool is_valid_cell_pointer(Cell const* cell)
+    {
+        return cell_from_possible_pointer((FlatPtr)cell);
+    }
+
+    IntrusiveListNode<HeapBlock> m_list_node;
+
+    CellAllocator& cell_allocator() { return m_cell_allocator; }
+
+private:
+    HeapBlock(Heap&, CellAllocator&, size_t cell_size);
+
+    bool has_lazy_freelist() const { return m_next_lazy_freelist_index < cell_count(); }
+
+    struct FreelistEntry final : public Cell {
+        GC_CELL(FreelistEntry, Cell);
+
+        RawPtr<FreelistEntry> next;
+    };
+
+    Cell* cell(size_t index)
+    {
+        return reinterpret_cast<Cell*>(&m_storage[index * cell_size()]);
+    }
+
+    CellAllocator& m_cell_allocator;
+    size_t m_cell_size { 0 };
+    size_t m_next_lazy_freelist_index { 0 };
+    Ptr<FreelistEntry> m_freelist;
+    alignas(__BIGGEST_ALIGNMENT__) u8 m_storage[];
+
+public:
+    static constexpr size_t min_possible_cell_size = sizeof(FreelistEntry);
+};
+
+}