Add µPickle support

[tom91136]

Would be nice to see support for µPickle which looks like the following:

For case clasess:

import upickle.default.{ReadWriter => RW, macroRW}
case class Thing(myFieldA: Int, myFieldB: String)
object Thing{
  implicit val rw: RW[Thing] = macroRW
}
case class Big(i: Int, b: Boolean, str: String, c: Char, t: Thing)
object Big{
  implicit val rw: RW[Big] = macroRW
}

And for sealed traits:

sealed trait IntOrTuple
object IntOrTuple{
  implicit val rw: RW[IntOrTuple] = RW.merge(IntThing.rw, TupleThing.rw)
}
case class IntThing(i: Int) extends IntOrTuple
object IntThing{
  implicit val rw: RW[IntThing] = macroRW
}
case class TupleThing(name: String, t: (Int, Int)) extends IntOrTuple
object TupleThing{
  implicit val rw: RW[TupleThing] = macroRW
}