The registered fromVersions in ascending order.
The step upgrading from fromVersion, or undefined.
Whether a step upgrading from fromVersion is registered.
Registers migrate as the step from fromVersion to fromVersion + 1.
Returns this so registrations chain.
The registered upgrade steps (§80), keyed by the version each upgrades from.
An instance rather than a module-level singleton: migrations are application (and test) state, and a shared mutable chain would make one suite's registration visible to the next.