Forward references to effect operations aren't supported

This happens when using an effect (and has a somewhat confusing error message):
```effekt
def test() = {
def f: Foo = <>
f.foo() // Cannot resolve operation foo, called on a receiver that is a computation.
}

interface Foo {
def foo(): Unit
}
```
or implementing one:
```
def test() = {
val f = new Foo {
def foo() = <> // Operation foo is not part of interface Foo.
}
}

interface Foo {
def foo(): Unit
}
```