LibJS: Add "Heap" and "Runtime" subdirectories

Let's try to keep LibJS tidy as it expands. :^)

Libraries/LibJS/Heap/HeapBlock.h

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Runtime/Cell.h>
+
+namespace JS {
+
+class HeapBlock {
+public:
+    static constexpr size_t block_size = 16 * KB;
+    static NonnullOwnPtr<HeapBlock> create_with_cell_size(Heap&, size_t);
+
+    void operator delete(void*);
+
+    size_t cell_size() const { return m_cell_size; }
+    size_t cell_count() const { return (block_size - sizeof(HeapBlock)) / m_cell_size; }
+
+    Cell* cell(size_t index) { return reinterpret_cast<Cell*>(&m_storage[index * cell_size()]); }
+
+    Cell* allocate();
+    void deallocate(Cell*);
+
+    template<typename Callback>
+    void for_each_cell(Callback callback)
+    {
+        for (size_t i = 0; i < cell_count(); ++i)
+            callback(cell(i));
+    }
+
+    Heap& heap() { return m_heap; }
+
+    static HeapBlock* from_cell(const Cell* cell)
+    {
+        return reinterpret_cast<HeapBlock*>((FlatPtr)cell & ~(block_size - 1));
+    }
+
+private:
+    HeapBlock(Heap&, size_t cell_size);
+
+    struct FreelistEntry : public Cell {
+        FreelistEntry* next;
+    };
+
+    Heap& m_heap;
+    size_t m_cell_size { 0 };
+    FreelistEntry* m_freelist { nullptr };
+    u8 m_storage[];
+};
+
+}