Tighter alembic integration

[boydgreenfield]

It'd be nice if had better support for various items in Alembic including:

• Alter column operations (esp type changes) (PostgreSQL-Audit does something similar here)
• Column renames
• Table renames

These are mostly manual in Alembic, so this is probably an extra operation that users can invoke vs. something automated, though it'd be very neat to autogenerate it as that becomes an Alembic feature (it seems like compare_type can be passed but is False by default in Alembic env.py files).