Add ReadOnly iterators and refactor other iterators

[fxamacker]

Updates #292

Changes:

• Adds ReadOnly iterators that match current iterator API (except for the "ReadOnly" suffix added to some function names).
• Refactors API of non-Readonly iterators because register inlining will require more parameters for MapIterator.

For ReadOnly iterators, the caller is responsible for preventing changes to child containers during iteration because mutations of child containers are not guaranteed to persist.

For non-ReadOnly iterators, two additional parameters are needed to update child container in parent map when child container is modified.

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• [x] Targeted PR against main branch
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [x] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[turbolent]

@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?

[fxamacker]

I'm replacing this PR with a new one that supports mutation of values from non-readonly iterators. The replacement PR will also include the atree inlining changes from PR #342.

A followup issue will also be opened in onflow/cadence repo.

[fxamacker]

This PR is replaced by PR #345