Add Iterant.takeWhileWithIndex

[alexandru]

Similar with issue #496 we need a takeWhileWithIndex on Iterant that takes elements for as long as the given predicate returns true:

sealed abstract class Iterant[F[_], A] {
  // ...
  def takeWhileWithIndex(p: (A, Int) => Boolean)(implicit F: Sync[F]): Iterant[F, A]
}

The implementation should be very similar actually.

[greenhat]

I'd like to take this one.

[alexandru]

@greenhat sure, go for it