[Zion] Add more debug information about kernel heap usage.

This will be helpful as we work to improve it.

Without deallocations, we currently stand at the following alloc numbers
8 - 142
16 - 319
32 - 364

unsized - 305

total page usage including slabs is 12.

zion/memory/slab_allocator.h

@@ -19,6 +19,7 @@ class Slab : public glcr::RefCounted<Slab>,
   void Free(void* addr);
 
   bool IsFull();
+  uint64_t Allocations() { return num_allocated_; }
 
  private:
   struct FreeListEntry {
@@ -39,6 +40,10 @@ class SlabAllocator {
 
   glcr::ErrorOr<void*> Allocate();
 
+  // Stats:
+  uint64_t SlabCount();
+  uint64_t Allocations();
+
  private:
   uint64_t elem_size_;
   glcr::IntrusiveList<Slab> slabs_;