Fix, document and format existing demos

1. Dropped GenSet

The main change is that demos from lecture 3 now compile.

As GetSet is deprecated, I first tried to rewrite a simplified version for Scala 3 that we could use:

GenSet trait and conversions

package common

import scala.collection.parallel.immutable.ParSet
import collection.parallel.CollectionConverters.IterableIsParallelizable

/** GenSet trait to replace the trait deprecated in Scala 2.13. */
sealed trait GenSet[A] extends Iterable[A]:
  def +(elem: A): GenSet[A]
  def -(elem: A): GenSet[A]

  def contains(elem: A): Boolean
  def subsetOf(that: GenSet[A]): Boolean
  def iterator: Iterator[A]

  def apply(elem: A) = contains(elem)

final case class SetGenSet[A](set: Set[A]) extends GenSet[A]:
  def +(elem: A): GenSet[A] = set + elem
  def -(elem: A): GenSet[A] = set - elem

  def contains(elem: A): Boolean = set.contains(elem)
  def subsetOf(that: GenSet[A]): Boolean = set.subsetOf(that)
  def iterator: Iterator[A] = set.iterator

final case class ParSetGenSet[A](set: ParSet[A]) extends GenSet[A]:
  def +(elem: A): GenSet[A] = set + elem
  def -(elem: A): GenSet[A] = set - elem

  def contains(elem: A): Boolean = set.contains(elem)
  def subsetOf(that: GenSet[A]): Boolean = set.subsetOf(that)
  def iterator: Iterator[A] = set.iterator

given fromParSetToGenSet[A]: Conversion[ParSet[A], GenSet[A]] = ParSetGenSet(_)
given fromSetToGenSet[A]: Conversion[Set[A], GenSet[A]] = SetGenSet(_)

given fromGenSetToSet[A]: Conversion[GenSet[A], Set[A]] = _ match
  case SetGenSet[A](s)    => s
  case ParSetGenSet[A](s) => s.seq

given fromGenSetToParSet[A]: Conversion[GenSet[A], ParSet[A]] = _ match
  case SetGenSet[A](s)    => s.par.toSet
  case ParSetGenSet[A](s) => s

But this one is buggy: there are unwanted conversations or snapshots that defeat the purpose of the demos. Furthermore, I think it's more confusing to have a fake trait than none at all.

Therefore, in a second iteration, I just dropped all usages of GenSet. It's simpler and shorter, but preserve the points of the demos.

2. Added documentation and setup scalafmt

Also added a lot of comments, and setup scalafmt.