New Handle manager

This implements a new Handle manager, adds ref-counting to kernel objects and implements `DuplicateHandle`. Note that support for `DuplicateHandle` is still fragile, because the rest of the kernel doesn't expects object to be able to have multiple handles pointing to them. Regardless, it seems to work OK on what I tested so far. I plan to fix this next, but decided it would be good to submit PRs more often to reduce rebase pain and reduce review sizes.
Why not use BitField for `GetSlot` and `GetGeneration`?