runtime: make stringNext use byteindex only, fix index offset

Use stringIterator.byteindex as the loop index, and remove
stringIterator.rangeindex, as "the index of the loop is the starting
position of the current rune, measured in bytes".  This patch also fixes
the current loop index returned by stringNext, using `it.byteindex'
before - not after - `length' is added.

src/runtime/string.go

@@ -14,8 +14,7 @@ type _string struct {
 
 // The iterator state for a range over a string.
 type stringIterator struct {
-	byteindex  uintptr
-	rangeindex uintptr
+	byteindex uintptr
 }
 
 // Return true iff the strings match.
@@ -105,10 +104,10 @@ func stringNext(s string, it *stringIterator) (bool, int, rune) {
 	if len(s) <= int(it.byteindex) {
 		return false, 0, 0
 	}
+	i := int(it.byteindex)
 	r, length := decodeUTF8(s, it.byteindex)
 	it.byteindex += length
-	it.rangeindex += 1
-	return true, int(it.rangeindex), r
+	return true, i, r
 }
 
 // Convert a Unicode code point into an array of bytes and its length.