Loosen auto-migrate restrictions on enums: allow adding new variants at the tail

[gefjon]

Treat the variants of a sum type as an ordered list. Require that the pre-migration type is a prefix of the post-migration type, rather than exactly equal. Add a smoketest that it is possible to add a new enum variant in a publish without -c.

[gefjon]

Appears to trace to this equality check.

[Centril]

Was fixed by #2874.