Refactor AMT to use iter_mut Instead of for_each_mut

[Sahilgill24]

This PR refactors the AMT mplementation by replacing internal iteration via for_each_mut with external iteration using iter_mut and adds a test case showing the use case of the iter_mut function .

Motivation:

Solves #1996

Progress:

• The iter_mut function yields a Smart pointer Amtptr.
• Amtptr can be dereferenced immutably.
• Can be dereferenced mutably.
• Can be deleted to yield the inner value.

Note

Instead of using the deref_mut , the get_mut function can be used instead as written in the test case and as shown
below

    for ptr in new_amt.iter_mut() {
        let current_idx = ptr.0;
        let mut val = ptr.1;
        let mut val_ref = val.get_mut();
        f(current_idx, &mut ValueMut::new(&mut *val_ref)).unwrap();
    }

[Copilot]

Pull Request Overview

This PR refactors the AMT implementation by replacing internal mutable iteration (for_each_mut) with an external iterator (iter_mut) that yields a smart pointer (Amtptr), and adds a corresponding test case to validate the new behavior.

• Introduces the iter_mut function in the AMT implementation which returns mutable smart- pointers (Amtptr).
• Adds a new test ("iter_mutable") demonstrating the usage of the iter_mut function.
• Updates related module imports and minor adjustments in the value_mut module.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
ipld/amt/tests/amt_tests.rs	New test case for iter_mutable functionality added.
ipld/amt/src/value_mut.rs	Minor formatting improvement with additional import.
ipld/amt/src/amt.rs	Added Amtptr struct and iter_mut function, with associated changes.