Add missing Factory for IArray

[j-mie6]

The regular scala.Array object has a implicit def toFactory[A: ClassTag]: Factory[A, Array[A]] (with a dummy implicit for some, probably good but maybe Scala 2 related, reason).

It would be nice if the equivalent method was added to IArray's companion object too, as this would allow for generic algorithms that construct immutable arrays.

[j-mie6]

(I'm happy to do this if there is no good reason not to do so)

[bishabosha]

also the BuildFrom instance is missing : #19229