@:structInit meta callback argument

Since constructor does not exist at build macro stage, I want to suggest callback arg, that will be called at the end of generated constructor, so you don't need to write constructor signature just to init some complex fields.
```haxe
@:structInit(init)
class Foo {
public var a = 0;
public var b = 0;
public var sum:Int = null;

inline function init() {
this.sum = a + b;
}
}
```