Nicer error messages on a likely forgotten resume

[jiribenes]

Students (and many other users like me) often forget to resume from a handler.
We could customise the error message, especially when it's obvious from the type that using resume would fix the type error one gets.

Let's take a look at a small example:

effect get(): Int

def foo() = 
  try { println(do get()) }
  with get {
    42
//  ^^
//  Expected Unit but got Int.
  }

We could customise ( 🚲🏠 ) the error to something slightly more specific like:

Expected Unit but got Int.
Did you mean to use `resume: Int => Unit` in the handler in order to resume?

The heuristic is pretty straightforward: an additional hint should fire if in the handler we get "expected τ1 but got τ2" and resume has type τ2 => τ1.

[jiribenes]

If I'm looking at it correctly, the error message is produced by the body checkAgainst ret here (and we could use the resumeType):

effekt/effekt/shared/src/main/scala/effekt/Typer.scala

Lines 470 to 482 in 000a768

	// (4) synthesize type of continuation
	val resumeType = if (isBidirectional) {
	// resume { {f} => e }
	val resumeType = FunctionType(Nil, cparams, Nil, bps, tpe, effs)
	val resumeCapt = CaptureParam(Name.local("resumeBlock"))
	FunctionType(Nil, List(resumeCapt), Nil, List(resumeType), ret, Effects.Pure)
	} else {
	// resume(v)
	FunctionType(Nil, Nil, List(tpe), Nil, ret, Effects.Pure)
	}
	Context.bind(Context.symbolOf(resume).asBlockSymbol, resumeType, continuationCapt)
	body checkAgainst ret