Support invoking default method when implementing interfaces

Interfaces can have a `default` implementation for their methods. We should support invoking this default when implementing the interface.

Take `Consumer`:

```java
public interface Consumer {
void accept(T t);

default Consumer andThen(Consumer after) {
Objects.requireNonNull(after);
return (T t) -> { accept(t); after.accept(t); };
}
}
```

Ideally we would want `$ConsumerImpl`'s `andThen` parameter to be optional.