Support multiple operations acting on the same object in one migration

[Andrew Farries]

Migrations are composed of arbitrarily many operations, but with the limitation that a later operation in a migration cannot act on an object that has been created or modified by an earlier operation.

This limitiation severely impacts the usefulness of having multiple operations per migration.

It should be possible to compose arbitrary operations without this restriction.

[Andrew Farries]

This was raised in the specific case of creating a table then adding an index to it in #203.