This commit returns ReadOnlyIteratorElementMutationError when
elements of readonly map iterator are mutated. Also, a
callback can be provided by the caller to log or debug
such mutations with more context.

As always, mutation of elements from readonly iterators are
not guaranteed to persist.

Instead of relying on other projects not to mutate readonly elements,
this commit returns ReadOnlyIteratorElementMutationError when
elements from readonly iterators are mutated.

This commit also adds readonly iterator functions that receive
mutation callbacks.  Callbacks are useful for logging, etc. with
more context when mutation occurs.  Mutation handling is the same
with or without callbacks.  If needed, other projects using atree
can choose to panic in the callback when mutation is detected.

If elements from readonly iterators are mutated:
- those changes are not guaranteed to persist.
- mutation functions of child containers return
  ReadOnlyIteratorElementMutationError.
- ReadOnlyMapIteratorMutationCallback are called if provided