Fix SlabIterator to include nested storage ID (non-inlining feature branch)

[fxamacker]

Closes #397

This problem does not affect atree inlining feature branch.

SlabIterator is used by storage health check, which is used for tests and for new migration features like filtering out old unreferenced slabs, etc.

Currently, SlabIterator doesn't traverse deep enough to handle nested storage ID inside another storable such as Cadence SomeValue.

This PR updates SlabIterator to handle nested storage ID in element such as Cadence SomeValue.

---

• [ ] 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 Does this PR still need to get ported to another feature branch?

[fxamacker]

@fxamacker Does this PR still need to get ported to another feature branch?

@turbolent No, because this was already in the atree-inlining feature branch (I used this to iterate atree inlined slabs).