Convention for allocator and arena passing

[jnterry]

Arenas and allocators are currently passed by both reference and by pointer to different functions, and constantly dereferencing and taking the address of them is a pain.

Decide on a convention, probably by reference as we don't want to allow them to be null (but do we need to allocate them on the heap???), and add to check_code_style.hpp

We also want move assignment operators for all of them (recently added for ArenaPool, might not be in others)

[jnterry]

It's probably best to use pointers to pass allocators around, since we want to be able to store them in POD structs, and we cant store references in structs without having a constructor.

Arenas could be passed around by reference, since we just store the arena struct as a member of a POD struct directly, no need to chase pointers.

Eg,
struct Kernel { xen::Allocator* root_allocator; xen::ArenaPool<LoadedModule> modules; };