Use atree's new read-only iterators when mutation of returned values are not required

[fxamacker]

Issue to be solved

New read-only iterators will be available in atree after onflow/atree#345 is merged.

Using the read-only iterators will be more efficient, etc. for use cases that don't need to mutate returned values.

Context

@fxamacker Could you please open a tech-debt follow-up issue in the Cadence repo, so that we don't forget to switch Cadence to the new read-only versions where possible?

Originally posted by @turbolent in onflow/atree#329 (comment)

Suggested Solution

Use atree's new read-only iterators when mutation of returned values are not required.

[fxamacker]

Closed by #3170