Warn about usage of `List` operations that have a linear cost

> What is the type of decodedName before the cast?

Here is the type error I was getting 

    type mismatch; found : Name.this.ThisNameType required: LinearOperationsOnList.this.context.universe.Name

Anyway, just mentioning it in case you are/were curious. Encoding the names makes more sense, and also removes the typing problem.