More warnings about unused effects

We should consider warning users when an effect is not used. I can think of the following two situations where we might want additional warnings:

```
def foo(): Int / emit[Int] = 42
```

Here we annotated that we use an effect but don't.

```
def bar() = boundary { () }
```

Here we use a "handler function" `boundary` which handles a `stop` effect but don't actually use the handled effect `stop`.