Add a migration that transforms dummy code CIDs to actual CIDs based on the actor code

[arajasek]

There's something of an open question about _where_this migration should go. I think it should just be in specs-actors -- the last thing that repo can do is transform the state tree to create the new version for the FVM-based future to use :)

[jennijuju]

migration of the v7 -> v8 CID filecoin-project/specs-actors#1576

as a part of the filecoin-project/builtin-actors#4 we will get the new CID into the migration

[raulk]

Implemented and landed in nv16 branch.