SLC: Add groupBy(mapFactory)(f) overload to IterableOps and ArrayOps

Scala Standard Library
1 
def groupBy[K, CC[_, _]](mapFactory: MapFactory[CC])(f: A => K): CC[K, C]
def groupBy[K, CC[_, _]](mapFactory: MapFactory[CC])(f: A => K): CC[K, Array[A]]
def groupBy[K, CC[_, _]](mapFactory: MapFactory[CC])(f: A => K): CC[K, IArray[A]]

And then ,the usage can be

    1 import scala.collection.immutable.{ListMap, VectorMap}                                                                                                                                
    2                                                                                                                                                                                       
    3 val xs = List(3, 1, 4, 1, 5, 9, 2, 6)                                                                                                                                                 
    4 val grouped: ListMap[Int, List[Int]] = xs.groupBy(ListMap)(_ % 3)                                                                                                                     
    5 //   ListMap(0 -> List(3, 9, 6), 1 -> List(1, 1, 4), 2 -> List(5, 2))                                                                                                                 
    6 //   keys appear in first-seen order: 3 (key 0), 1 (key 1), 5 (key 2)
2

Could you explain a bit more what this change does, and what is the motivation behind it ?

3

The current groupBy returns a Map whose iterating order is undefined. I want to have a SeqMap which keeps the original order.