Add EfiBox - RAII wrapper for heap ptrs

[gurry]

Currently we hold a lot of heap allocated pointers which we're just dropping on the floor when done with them. To fix that we should expose a type called EfiBox which is very similar to Box, but is guaranteed to go through our allocator or call UEFI's FreePool directly even. Will have to decide how much Boxs API surface we should copy.

[gurry]

Implemented

[gurry]

Implemented but not yet being used in places like DevicePath. Fix that as well.