Physical memory allocator improvement

[vercas]

It is vital that Beelzebub begins to use 2-MiB pages where possible.
These have their own TLBs in L1 and share entries with 4-KiB pages in L2.
Therefore, using 2-MiB pages allows threads and cores to handle significantly larger working sets, which is handy for both the kernel and especially userland.

This requires support from the PMM, which needs to provide aligned pages.

Thus, the following must be implemented:

Plans

• [x] Simplify and abstract away interface of the PMM; no need for consumers to know every little detail.
  • [ ] Expose simplified PMM to kernel modules.
• [x] Change physical allocation space to work with 2-MiB frames.
  • [x] Serve 4-KiB frames from within 2-MiB frames. Management structures for the 4-KiB frames should be within the 2-MiB frame.
  • [x] Use frame dangling at the end of the allocation space, if any.
  • [ ] Use frame at the start of the allocation space, if any.

[vercas]

This design does not allow lock-free allocation, so the related task was removed from the issue.
It does, however, allow a finer level of locking than the previous design.

Also, working on all features at once. Much easier this way. Will take a while.

[vercas]

Rowhammer-proofing is a book-keeping nightmare, and fails when shared memory is involved.
Will not implement.

[vercas]

Reference counting is not fully implemented yet.

[vercas]

Reference counting is fully implemented.
God help me.