Replace `for_each_mut` (internal iteration) with `iter_mut` (external iteration) in the AMT

The iterator will have to yield a smart pointer that:

1. Can be dereferenced immutably with no effect.
2. Can be dereferenced mutably, marking the value as changed.
3. Can be deleted (e.g., by calling some form of `ptr.delete()` method). This method should _consume_ the smart pointer (possibly returning the inner value).

This will get rid of the last "internal" iteration feature.