Require the Each keyword to flatMap to a collection type

[Atry]

val y: Vector[Int] = {
  val x: Int = !Each(List(1, 2))
  Vector(x + 10) // Good
}

val y: Int = {
  val x: Int = !Each(List(1, 2))
  x + 10 // this previously compiles but it is invalid since this commit, because x + 10 is not a collection
}