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/BlockAllocator.cpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2021-2023, Andreas Kling <andreas@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Platform.h>
+#include <AK/Random.h>
+#include <AK/Vector.h>
+#include <LibGC/BlockAllocator.h>
+#include <LibGC/HeapBlock.h>
+#include <sys/mman.h>
+
+#ifdef HAS_ADDRESS_SANITIZER
+#    include <sanitizer/asan_interface.h>
+#    include <sanitizer/lsan_interface.h>
+#endif
+
+#if defined(AK_OS_GNU_HURD) || (!defined(MADV_FREE) && !defined(MADV_DONTNEED))
+#    define USE_FALLBACK_BLOCK_DEALLOCATION
+#endif
+
+namespace GC {
+
+BlockAllocator::~BlockAllocator()
+{
+    for (auto* block : m_blocks) {
+        ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
+        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()) {
+        // To reduce predictability, take a random block from the cache.
+        size_t random_index = get_random_uniform(m_blocks.size());
+        auto* block = m_blocks.unstable_take(random_index);
+        ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
+        LSAN_REGISTER_ROOT_REGION(block, HeapBlock::block_size);
+        return block;
+    }
+
+    auto* block = (HeapBlock*)mmap(nullptr, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+    VERIFY(block != MAP_FAILED);
+    LSAN_REGISTER_ROOT_REGION(block, HeapBlock::block_size);
+    return block;
+}
+
+void BlockAllocator::deallocate_block(void* block)
+{
+    VERIFY(block);
+
+#if defined(USE_FALLBACK_BLOCK_DEALLOCATION)
+    // If we can't use any of the nicer techniques, unmap and remap the block to return the physical pages while keeping the VM.
+    if (munmap(block, HeapBlock::block_size) < 0) {
+        perror("munmap");
+        VERIFY_NOT_REACHED();
+    }
+    if (mmap(block, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0) != block) {
+        perror("mmap");
+        VERIFY_NOT_REACHED();
+    }
+#elif defined(MADV_FREE)
+    if (madvise(block, HeapBlock::block_size, MADV_FREE) < 0) {
+        perror("madvise(MADV_FREE)");
+        VERIFY_NOT_REACHED();
+    }
+#elif defined(MADV_DONTNEED)
+    if (madvise(block, HeapBlock::block_size, MADV_DONTNEED) < 0) {
+        perror("madvise(MADV_DONTNEED)");
+        VERIFY_NOT_REACHED();
+    }
+#endif
+
+    ASAN_POISON_MEMORY_REGION(block, HeapBlock::block_size);
+    LSAN_UNREGISTER_ROOT_REGION(block, HeapBlock::block_size);
+    m_blocks.append(block);
+}
+
+}