Abstract as IntMap key prevents unification with Iterable<V>

This is quite awkward, not sure why it happens:

```haxe
using Lambda;

function main() {
[0 => 1].count();
[(0 : Foo) => 1].count(); // haxe.ds.Map has no field count
}

abstract Foo(Int) from Int {}
```

It works with a `to Int`.