compiler,runtime: implement string to []rune conversion

Commit message by aykevl

testdata/string.go

@@ -0,0 +1,19 @@
+package main
+
+func testRangeString() {
+	for i, c := range "abcü¢€𐍈°x" {
+		println(i, c)
+	}
+}
+
+func testStringToRunes() {
+	var s = "abcü¢€𐍈°x"
+	for i,c := range []rune(s) {
+		println(i, c)
+	}
+}
+
+func main() {
+	testRangeString()
+	testStringToRunes()
+}