ordered union type conversions

[dherman]

Converting to a union type needs to search the type in order.

use namespace intrinsic;
class Foo {
    static var FOO : Foo = new Foo;
    meta public static function convert(n : int) : Foo FOO
    public function toString() : string "Foo"
}
class Bar {
    static var BAR : Bar = new Bar;
    meta public static function convert(n : int) : Bar BAR
    public function toString() : string "Bar"
}
var x : (Foo, Bar) = 42
var y : (Bar, Foo) = 42
print(x) // "Foo"
print(y) // "Bar"