Add some short-cut for delegating serializers

[kunyavskiy]

What is your use-case and why do you need this feature?

Delegating serializers are common feature, but it's quite boilerplatish to use. Maybe we can implement some API, which would provide shorter syntax to create them.

Describe the solution you'd like

E.g. it can be a class you can extend, like.

abstract class DelegatingKSerializer<S, T> : KSerializer<T>(val delegate: KSerializer<S>) {
   override val descriptor get() = delegate.descriptor
   abstract fun serialize(value: T): S
   abstract fun deserialize(value: S): T
   override fun serialize(encoder: Encoder, value: T) =  encoder.encodeSerializableValue(delegateSerializer, serialize(value))
   override fun deserialize(decoder: Decoder): S = deserialize(decoder.decodeSerializableValue(delegateSerializer))
} 

Then example from tutorial would look like

class ColorIntArraySerializer : DelegatingKSerializer<Int, Color>(serializer()) {
    override fun serialize(value: Color) = intArrayOf(
            (value.rgb shr 16) and 0xFF,
            (value.rgb shr 8) and 0xFF,
            value.rgb and 0xFF
    )

    override fun deserialize(array: IntArray) = Color((array[0] shl 16) or (array[1] shl 8) or array[2])
}

which is significantly cleaner, than original one.

It would be also nice to avoid passing serializer() explicitly here, but I didn't come up how we can do this.