Kernel: Make kmalloc heap expansion kmalloc-free

Previously, the heap expansion logic could end up calling kmalloc
recursively, which was quite messy and hard to reason about.

This patch redesigns heap expansion so that it's kmalloc-free:

- We make a single large virtual range allocation at startup
- When expanding, we bump allocate VM from that region
- When expanding, we populate page tables directly ourselves,
  instead of going via MemoryManager.

This makes heap expansion a great deal simpler. However, do note that it
introduces two new flaws that we'll need to deal with eventually:

- The single virtual range allocation is limited to 64 MiB and once
  exhausted, kmalloc() will fail. (Actually, it will PANIC for now..)

- The kmalloc heap can no longer shrink once expanded. Subheaps stay
  in place once constructed.

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