LibJS: Recycle up to 64 HeapBlocks to improve performance :^)

This patch adds a BlockAllocator to the GC heap where we now cache up to
64 HeapBlock-sized mmap's that get recycled when allocating HeapBlocks.

This improves test-js runtime performance by ~35%, pretty cool! :^)

Userland/Libraries/LibJS/Heap/BlockAllocator.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibJS/Forward.h>
+
+namespace JS {
+
+class BlockAllocator {
+public:
+    BlockAllocator();
+    ~BlockAllocator();
+
+    void* allocate_block(char const* name);
+    void deallocate_block(void*);
+
+private:
+    static constexpr size_t max_cached_blocks = 64;
+
+    Vector<void*> m_blocks;
+};
+
+}