Move toString() from primitive ranges to Range

The same can't be done with progressions, because its toString() checks if an
increment is greater than zero, but Progression.increment is of type Number,
which is not Comparable<Int>

libraries/stdlib/src/kotlin/Ranges.kt

@@ -7,10 +7,6 @@ public data class ComparableRange<T: Comparable<T>> (
     public override fun contains(item: T): Boolean {
         return start <= item && item <= end
     }
-
-    public fun toString(): String {
-        return "$start..$end"
-    }
 }
 
 public fun <T: Comparable<T>> T.rangeTo(that: T): ComparableRange<T> {