Better `Dynamic*` mutable iteration

## What problem does this solve or what need does it fill?

Mutating all fields/key-value pairs/entries/etc. in a dynamic something isn't "easily" doable yet.

## What solution would you like?

Add `IterFieldsMut` next to `IterFields` and for all the other things too.

## What alternative(s) have you considered?

Writing it my own.

## Additional context

It's not as easy to do as the non-mutable ones, because of lifetimes. We must enforce that an index can't reference the same object and trix with unsafe code.