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.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Badge.h>
+#include <LibJS/Heap/BlockAllocator.h>
+#include <LibJS/Heap/CellAllocator.h>
+#include <LibJS/Heap/Heap.h>
+#include <LibJS/Heap/HeapBlock.h>
+
+namespace JS {
+
+CellAllocator::CellAllocator(size_t cell_size)
+    : m_cell_size(cell_size)
+{
+}
+
+CellAllocator::~CellAllocator()
+{
+}
+
+Cell* CellAllocator::allocate_cell(Heap& heap)
+{
+    if (m_usable_blocks.is_empty()) {
+        auto block = HeapBlock::create_with_cell_size(heap, m_cell_size);
+        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)
+{
+    auto& heap = block.heap();
+    block.m_list_node.remove();
+    // NOTE: HeapBlocks are managed by the BlockAllocator, so we don't want to `delete` the block here.
+    block.~HeapBlock();
+    heap.block_allocator().deallocate_block(&block);
+}
+
+void CellAllocator::block_did_become_usable(Badge<Heap>, HeapBlock& block)
+{
+    VERIFY(!block.is_full());
+    m_usable_blocks.append(block);
+}
+
+}