easyjson:skip.

we don't want the user to have the option to call `Be<Type>` on *anyOf<> types.

we don't want the user to have the option to call `Be<Type>` on []anyOf<> types, as elements in slices are listed by their IDs (in this case the ID of the underlying element) and there is no usecase where Be<Type> on an []anyOf<> type element is more convenient for the user than just deleting and creating a new slice element.