Ignored type arguments

It would be nice to have ignored type arguments like in Scala `Foo[_]` or in Kotlin `Foo<*>`. This can be useful for interfaces where an operation does not use one of the interface's type parameters, and hence callers don't have to have it at hand. This basically opens the door to existential types as well.
Example:
```effekt
interface Foo[A, B] {
def foo(): A
}
def foo[A](): A / Foo[A, _] = do foo()
```