write xml

Hello,

This PR add Writes to validation-xml.

Example usage:

```scala
val w = To[XmlWriter] { __ => (
  (__ \ "firstname").write[String] ~
  (__ \ "age").write[Int]
) tupled
}
w.writes("Julien", 28)(<user></user>) mustEqual <user><firstname>Julien</firstname><age>28</age></user>

val w1 = To[XmlWriter] { __ => (
  attributeW[String]("firstname") ~
  attributeW[Int]("age")
) tupled
}
w1.writes("Julien", 28)(<user></user>) mustEqual <user firstname="Julien" age="28"></user>

val w2 = To[XmlWriter] { __ => (
  attributeW[String]("firstname") ~
  (__ \ "age").write[Int]
) tupled
}
w2.writes("Julien", 28)(<user></user>) mustEqual <user firstname="Julien"><age>28</age></user>
```