support covariance for abstracts

I know variance a big and confusing topic, but since we allow covariance for structures (e.g. passing `Array` as `Iterable`), maybe we could also allow it for abstracts (to pass `Array` to `ReadOnlyArray`)? Also ideally we would treat interfaces just like structs wrt variance.

```haxe
class Parent {
public function new() {}
}

class Child extends Parent {}

function withStruct(arr:Iterable) {}

function withAbstract(arr:haxe.ds.ReadOnlyArray) {}

function main() {
// this works
withStruct(new Array());
// this doesn't
withAbstract(new Array());
}
```

I guess the biggest challenge here is to tame target native compilers?