Free p[123] table if empty in `mapper::unmap()`

[4e554c4c]

I've been going through the TODOs and found this issue
https://github.com/ESALP/ESALP-1/blob/master/src/memory/paging/mapper.rs#L148

[4e554c4c]

I've about solved this, but got into an argument on #osdev about whether it was a good idea to do. unmap should be happening quite often in the kernel, so maybe we should do the minnimum needed there and free page tables later in a worker thread (this would be part of the vmm).

If this is wanted now I can submit my changes, otherwise we can wait until it'd be better to do.