Generic instances for value classes

It would be great if these would make it to the library. Happy to open a PR for it, if somebody points me to the right place for these:

  implicit def decoderValueClass[T <: AnyVal, V](
    implicit
    gen:     Lazy[Generic.Aux[T, V :: HNil]],
    decoder: Decoder[V]
  ): Decoder[T] = Decoder.instance { cursor ⇒
    decoder(cursor).map { value ⇒
      gen.value.from(value :: HNil)
    }
  }

  implicit def encoderValueClass[T <: AnyVal, V](
    implicit
    gen:     Lazy[Generic.Aux[T, V :: HNil]],
    encoder: Encoder[V]
  ): Encoder[T] = Encoder.instance { value ⇒
    encoder(gen.value.to(value).head)
  }

[pjan]

@Tvaroh not a discussion that we need to have here (I don’t want to highjack the conversation), but not everything needs to be heavily speed/memory optimized, and valueclasses can be beneficial as they can eliminate a whole class of developer bugs in environments where these can be particularly costly.

[melrief]

@Tvaroh nothing in this PR forces the developer to use value classes but for people who may want to use them this can be useful. I don’t think this is the right place to have a discussion about a feature of the Scala language.