Reverse direction of control on Migratable#migrate

Right now, a migratable object migrates itself using a migrator. It would probably make more sense to overload Migrator#migrate so that it can take either a migration name and a block or a migratable object.

Start by reading Migratable#migrate and Migrator#migrate to understand the current call direction and the existing migration-name-and-block form. Define the desired Migrator#migrate API for accepting either a migratable object or a migration name with a block, then verify that both forms preserve the expected migration behavior.

Written by the indexing model from the issue text.