Prepare builtin actor migrations

[arajasek]

For the v15-v16 migration, we will be migrating FROM baked-in synthetic CIDs TO "real" CIDs passed in through the manifest. This work is already in review.

v16-v17 migrations and onwards will need a slightly different approach -- we will migrate from CIDs that we read out of the system actor to CIDs passed in through the manifest. It is advisable to have this migration written ahead of M1 (even though it is not needed for M1) to be prepared for a hasty upgrade if needed.