cleanup the slab types

- get rid of HeaderOrIdxPtr
- add IndexPtrSlab
- add UntypedArenaSlab
- add to_untyped for converting a typed slab into an unsyped slab
- remove TypedArenaPtr::new, as they shouldn't be created outside of this module

src/types.rs

@@ -710,6 +710,7 @@ impl UntypedArenaPtr {
 
     /// # Safety
     /// - this UntypedArenaPtr actuall pointee type is T
+    /// - the pointer must be non-null
     #[inline]
     pub unsafe fn as_typed_ptr<T: ?Sized + ArenaAllocated>(self) -> TypedArenaPtr<T>
     where