refactor: consolidate allocators

[DonIsaac]

What does this PR do?

Moves all memory allocators into src/allocators. The following allocators are affected:

• NullableAllocator
• MimallocArena
• MaxHeapAllocator
• LinuxMemFdAllocator
• All allocators in memory_allocator.zig (CAllocator, ZAllocator, HugeAllocator, etc)

I also added an underflow check to LinuxMemFdAllocator in case a consumer calls deref() without first calling ref()

[robobun]

^{Updated 2:39 PM PT - Jan 8th, 2025}

✅ @DonIsaac, your commit 1e0954b has passed in #9254! 🎉

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🧪   try this PR locally:

bunx bun-pr 16061

[dylan-conway]

Just one comment

[dylan-conway]

Looks good