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.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Vector.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Heap/BlockAllocator.h>
+#include <LibJS/Heap/HeapBlock.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+
+namespace JS {
+
+BlockAllocator::BlockAllocator()
+{
+}
+
+BlockAllocator::~BlockAllocator()
+{
+    for (auto* block : m_blocks) {
+        if (munmap(block, HeapBlock::block_size) < 0) {
+            perror("munmap");
+            VERIFY_NOT_REACHED();
+        }
+    }
+}
+
+void* BlockAllocator::allocate_block([[maybe_unused]] char const* name)
+{
+    if (!m_blocks.is_empty())
+        return m_blocks.take_last();
+
+#ifdef __serenity__
+    auto* block = (HeapBlock*)serenity_mmap(nullptr, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_RANDOMIZED | MAP_PRIVATE, 0, 0, HeapBlock::block_size, name);
+#else
+    auto* block = (HeapBlock*)aligned_alloc(HeapBlock::block_size, HeapBlock::block_size);
+#endif
+    VERIFY(block != MAP_FAILED);
+    return block;
+}
+
+void BlockAllocator::deallocate_block(void* block)
+{
+    VERIFY(block);
+    if (m_blocks.size() >= max_cached_blocks) {
+        if (munmap(block, HeapBlock::block_size) < 0) {
+            perror("munmap");
+            VERIFY_NOT_REACHED();
+        }
+        return;
+    }
+
+    m_blocks.append(block);
+}
+
+}