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

What is the type of `decodedName` before the cast?

```
scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> def dec(s: Symbol) = s.name.decodedName
dec: (s: reflect.runtime.universe.Symbol)reflect.runtime.universe.Name
```

Incidentally, from a performance perspective, you would be actually better off encoding the names in the `Set` and just comparing these to them directly. Then you don't pay the cost of name manipulation for every `Select` node.