`unify_min` ignores `@:to` and `@:from` fields

Sample from https://github.com/HaxeFoundation/haxe/issues/5279#issuecomment-222106177
```haxe
class Main {
public static function main() {
function test() //:Int (works if explicitly hint return type as Int)
return if (true) getInt() else getAbstract();
}

static function getInt():Int return 1;
static function getAbstract():MyAbstract return "1";
}
abstract MyAbstract(String) from String to String {
@:to
public inline function toInt():Int return Std.parseInt(this);
}
```
Lines, which make the difference depending on return type hint:
https://github.com/HaxeFoundation/haxe/blob/e24798614b516420614b92163e649643fedb642e/src/typing/typer.ml#L2213-L2221
The same happens for `try...catch` and `switch` expressions.
I tried to change `Type.unify` to take into account `@:from`/`@:to`, but that breaks other things
Changing `unify_min` to handle `@:to/@:from` is also not enough, because (unlike `Abstract.cast_or_unify`) it does not return cast expressions.
I think we need to change `unify_min` and `unify_min_raise` to return expressions in addition to deduced type, but that will affect various other places in the compiler (e.g. array and map declaration)

Start with src/typing/typer.ml at the mentioned lines and trace unify_min, unify_min_raise, Type.unify, and Abstract.cast_or_unify. Use the Haxe sample as the reproduction; done means inferred return types handle @:to/@:from conversions consistently in the sample, try...catch, and switch expressions.

Written by the indexing model from the issue text.