Spec out much of the migration-checking flow.
Fix problem where we were not removing the unique index in the first place if :low_card_collapse_rows => true.
Add spec that we raise an error if there are too many rows in the low-card table.
Spec that :exclude_column_names works correctly.
Spec that we exclude :created_at and :updated_at by default.
Spec that we blow up correctly if trying to say has_low_card_table to something that isn't a low-card table.
Spec that low_card_collapsing_value_update_scheme :none works; spec that low_card_collapse_rows_and_update_referrers! returns a winners-to-losers map.