Using objects for handler implementations

As mentioned in #365 ([here](https://github.com/effekt-lang/effekt/issues/395#issuecomment-1961690461)), it should be possible to use objects as handler implementations.

```scala
// ...

def handler = new Handler {
def catch[R]{ p: => R / Exc } { h: String => R } =
try { p() } with Exc {
def raise[A](msg) = h(msg)
}
}

def main() = try { p() } with handler;
```

Currently, this is not possible but should be for tail-resumptive operations.