Finally finished fixing the PMM free method!

After needing to quickly relearn the logic of the free method because I somehow forgot it, it finally works again!

Additionally, I’ve added a little safeguard to the bootloader memory reclamation. Next, I’ll be continuing on the VMM and refactoring it to support userland page tables too.

Also, I’ve added a few core::fmt traits to PhysAddr and VirtAddr and also added a constructor which creates a new address type (PhysAddr or VirtAddr) with an address of zero. In addition, PhysAddr got a few new getters like as_ptr or as_mut_ptr.