Kernel: Slap UNMAP_AFTER_INIT on a whole bunch of functions

There's no real system here, I just added it to various functions
that I don't believe we ever want to call after initialization
has finished.

With these changes, we're able to unmap 60 KiB of kernel text
after init. :^)

Kernel/Heap/SlabAllocator.cpp

@@ -141,7 +141,7 @@ void for_each_allocator(Callback callback)
     callback(s_slab_allocator_128);
 }
 
-void slab_alloc_init()
+UNMAP_AFTER_INIT void slab_alloc_init()
 {
     s_slab_allocator_16.init(128 * KiB);
     s_slab_allocator_32.init(128 * KiB);