More warnings about unused effects

[phischu]

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.