toInterface and unsafeToTemplate extension methods

[S11001001]

Design #13668 (comment)

• contract IDs
  • toInterface in object Template
  • unsafeToTemplate in object Interface (see generate uninhabited types for interfaces, with their choices, and resolve their contract IDs #13924)
• Template.Key, toInterface only
• Template.CreateForExercise, toInterface only
• introduce Template.Implements
• codegen Template.Implements instances, depends on generate uninhabited types for interfaces, with their choices, and resolve their contract IDs #13924