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.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2020-2023, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Badge.h>
+#include <LibGC/BlockAllocator.h>
+#include <LibGC/CellAllocator.h>
+#include <LibGC/Heap.h>
+#include <LibGC/HeapBlock.h>
+
+namespace GC {
+
+CellAllocator::CellAllocator(size_t cell_size, char const* class_name)
+    : m_class_name(class_name)
+    , m_cell_size(cell_size)
+{
+}
+
+Cell* CellAllocator::allocate_cell(Heap& heap)
+{
+    if (!m_list_node.is_in_list())
+        heap.register_cell_allocator({}, *this);
+
+    if (m_usable_blocks.is_empty()) {
+        auto block = HeapBlock::create_with_cell_size(heap, *this, m_cell_size, m_class_name);
+        auto block_ptr = reinterpret_cast<FlatPtr>(block.ptr());
+        if (m_min_block_address > block_ptr)
+            m_min_block_address = block_ptr;
+        if (m_max_block_address < block_ptr)
+            m_max_block_address = block_ptr;
+        m_usable_blocks.append(*block.leak_ptr());
+    }
+
+    auto& block = *m_usable_blocks.last();
+    auto* cell = block.allocate();
+    VERIFY(cell);
+    if (block.is_full())
+        m_full_blocks.append(*m_usable_blocks.last());
+    return cell;
+}
+
+void CellAllocator::block_did_become_empty(Badge<Heap>, HeapBlock& block)
+{
+    block.m_list_node.remove();
+    // NOTE: HeapBlocks are managed by the BlockAllocator, so we don't want to `delete` the block here.
+    block.~HeapBlock();
+    m_block_allocator.deallocate_block(&block);
+}
+
+void CellAllocator::block_did_become_usable(Badge<Heap>, HeapBlock& block)
+{
+    VERIFY(!block.is_full());
+    m_usable_blocks.append(block);
+}
+
+}