Migrations should be able to report *why* it is migrating

[parsonsmatt]

Migrations will report "what to do" in order to bring something up to spec, but they don't report "why."

For some casees, (ie add column or create table) this is straightforward - the thing didn't exist previously. But for alter, we should have enough information to determine why a migration is necessary - "the constraint is spelled wrong" is distinct from "the types are wrong."