Support for polymorphic deserializers base on tag name

Suppose we have a sealed class hierarchy like this:

sealed class S

@SerialName("a")
class A: Parent()

@SerialName("b")
class b: Parent()

class Holder{
  val children = ArrayList<@ContextualSerialization S>()
}

Then we create an array of S and pass it to serializer via `ContextSerializer, obtaining something like

<Holder>
  <a>...</a>
  <b>...</b>
</Holder>

Currently deserializer tries to interpret those tags as fields. It is important to be able to somehow tell it that those are parts of the list.

[altavir]

Seems to be working now!

[pdvrieze]

I’ve added some documentation to the README. I’ll keep @XmlPolyChildren as is for now. The main use case with the new autoPolymorphism is where it is necessary to specify the use tag name at use site (not on the type - something incompatible with normal use of XmlSchema). I’m not sure that class arrays are supported by the serialization plugin yet.