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

[Stebalien]

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.

[PhantomOz]

can i work on this?

[Stebalien]

Absolutely! Take a look at how we currently use it here.

[rjan90]

Hey @PhantomOz! Are you still interested in working on this, or should I unassign you?