slab allocator

[hexagonal-sun]

• libkernel: memory: reorganise allocators
  Create a new submodule within memory, allocators which contains all
  memory allocators. Also split out the Frame struct from the pg_alloc
  module, allowing it to be used by other modules.

• libkernel: memory: allocs: move FrameList impl into frame
  Move the FrameList logic out of the physical memory allocator into its
  own module. This allows FrameList to be shared between the physical
  memory allocator and the slab allocator.

• libkernel: memory: PageAllocGetter: return ref to allocator
  Rather than returning a reference to the OnceCell wrapping the
  allocator, return a static reference to the allocator itself. This
  allows flexibility for how the allocator is wrapped.

• libkernel: memory: slab: slab: new
  Add a new module for managing objects within a slab. The Slab struct
  manages objects of a given set within a contiguous set of pages. It is
  the handle to the underlying memory, allowing for objects to be
  allocated, and free'd. It manages a free list of 'indexes' within the
  object slots themselves.

• libkernel: memory: slab: allocator: new
  Add a new slab allocator implementation. The SlabAllocator manages
  lists of slabs for each size class.