Provide char-based Stepper for String as the default Stepper

Both `charStepper` and `codepointStepper` are available through
separate methods. The default `stepper` methods from `MakesIntStepper`
is equivalent to `charStepper`.

Author: szeiger

src/main/scala/scala/compat/java8/converterImpl/StepsString.scala

@@ -11,6 +11,12 @@ import Stepper._
 // Stepper implementation //
 ////////////////////////////
 
+private[java8] class StepperStringChar(underlying: String, _i0: Int, _iN: Int)
+  extends StepsIntLikeIndexed[StepperStringChar](_i0, _iN) {
+  def nextInt() = if (hasNext()) { val j = i0; i0 += 1; underlying.charAt(j) } else throwNSEE
+  def semiclone(half: Int) = new StepperStringChar(underlying, i0, half)
+}
+
 private[java8] class StepperStringCodePoint(underlying: String, var i0: Int, var iN: Int) extends IntStepper with EfficientSubstep {
   def characteristics() = NonNull
   def estimateSize = iN - i0
@@ -40,5 +46,7 @@ private[java8] class StepperStringCodePoint(underlying: String, var i0: Int, var
 /////////////////////////
 
 final class RichStringCanStep(private val underlying: String) extends AnyVal with MakesIntStepper {
-  @inline def stepper: IntStepper with EfficientSubstep = new StepperStringCodePoint(underlying, 0, underlying.length)
-}    
+  @inline def stepper: IntStepper with EfficientSubstep = charStepper
+  @inline def charStepper: IntStepper with EfficientSubstep = new StepperStringChar(underlying, 0, underlying.length)
+  @inline def codepointStepper: IntStepper with EfficientSubstep = new StepperStringCodePoint(underlying, 0, underlying.length)
+}