Generate fromProduct in the companion object (Scala 3)

[sideeffffect]

In Scala 3, there exists a method fromProduct that gets automatically generated in case classes' companion objects. For backwards compatibility, this methods needs to be explicitly re-implemented and needs to accept tuples of all the necessary arities.

The generated should look something like this

def fromProduct(p: Product): Person = p.productArity match
  case 2 =>
    Person(
      p.productElement(0).asInstanceOf[...],
      p.productElement(1).asInstanceOf[...],
    )
  case 3 =>
    Person(
      p.productElement(0).asInstanceOf[...],
      p.productElement(1).asInstanceOf[...],
      p.productElement(2).asInstanceOf[...],
    )
  ...

Context:

• https://contributors.scala-lang.org/t/can-we-make-adding-a-parameter-with-a-default-value-binary-compatible/6132/29?u=sideeffffect
• Improve the "Backwards compatible case classes" guide with Scala 2 and 3 specifics scala/docs.scala-lang#2788
• Add case classes for Ember client and server configuration http4s/http4s#7066 (comment)

/cc @armanbilge

[lihaoyi]

Done