Introduce `MaybeValidated` type to remove invalid states

The old return type (RecompRequired, Maybe _), was confusing because it was inhabited by values like (UpToDate, Nothing) that made no sense.

The new type ensures:

• you must provide a value if it is up to date.

• you must provide a reason if you don't provide a value.

it is used as the return value of:

• checkOldIface
• checkByteCode
• checkObjects