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.

testdata/rangestring.go

@@ -0,0 +1,11 @@
+package main
+
+func testRangeString() {
+	for i, c := range "abcü¢€𐍈°x" {
+		println(i, c)
+	}
+}
+
+func main() {
+	testRangeString()
+}

testdata/rangestring.txt

@@ -0,0 +1,9 @@
+0 97
+1 98
+2 99
+3 252
+5 162
+7 8364
+10 66376
+14 176
+16 120