HList.fill / Tuple.fill

This PR proposes to add a fill method on the HList singleton and in a Tuple singleton, that can be used to build HList or tuples. It allows one to write:

    Tuple.fill[_3]("value") :+ 2

or

    Tuple.fill[_4][Option[String]](None) ++ ("a", 2) // explicitly provided upper bound instead of None.type that would have been infered

Same thing for HList.fill.

I especially found it useful while writing code that looks like:

    val p: List[...]
    p.foldLeft(Tuple.fill[_3](Option.empty[String])) {
    // instead of p.foldLeft((Option.empty[String], Option.empty[String], Option.empty[String]))
      case ((None, None, None), item) =>
       ...
      case ... =>
    }

or

    p.foldLeft(Tuple.fill[_3][Either[List[String]]](Right(Nil))) {
    // instead of p.foldLeft((Right(Nil), Right(Nil), Right(Nil)): (Either[List[String]], Either[List[String]], Either[List[String]]))
      case (Right(acc1), Right(acc2), Right(acc3)) =>
        ...
      case ... =>
    }

Comments/critics welcome :-)