Fix migration filter for old unreferenced slabs (non-inlining feature branch)

[fxamacker]

Closes #395

Recently, migration programs in onflow/flow-go added a flag to filter out old unreferenced slabs and onflow/atree added some functions to support that. However, some of the old unreferenced slabs are not filtered.

This issue only affects this new migration feature in onflow/flow-go:

• https://github.com/onflow/flow-go/pull/5653
• :thread: https://discord.com/channels/613813861610684416/1232975765847543839/1233068490030846023
• https://github.com/dapperlabs/cadence-internal/issues/187

This PR fixes the migration filter by handling nested storage ID inside element such as Cadence SomeValue.

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