track data type dependencies

[agentm]

Removing a data type from the db context can invalidate a relvar's consistency.

TutorialD (master/main): data HairColor = Brown | Black | Red | Blond
TutorialD (master/main): salon_customer := relation{tuple{name "Bob", hair Brown}, tuple{name "Steve", hair Blond}}
TutorialD (master/main): undata HairColor
TutorialD (master/main): :showexpr salon_customer
ERR: NoSuchDataConstructorError "Brown"

We should prevent the deletion of a type which is in use in a relvar or any functions. They will need to be redefined.
In the future, we should define which alterations to a type are "safe" for existing data- for example, adding a value constructor to an existing data type should be safe for relvars, but could create unmatched pattern errors in database functions. Perhaps there is no safe way to alter database types.