compiler,runtime: implement []rune to string conversion

This is used by a few packages in the standard library, at least
compress/gzip and regexp/syntax.

testdata/string.go

@@ -8,12 +8,17 @@ func testRangeString() {
 
 func testStringToRunes() {
 	var s = "abcü¢€𐍈°x"
-	for i,c := range []rune(s) {
+	for i, c := range []rune(s) {
 		println(i, c)
 	}
 }
 
+func testRunesToString(r []rune) {
+	println("string from runes:", string(r))
+}
+
 func main() {
 	testRangeString()
 	testStringToRunes()
+	testRunesToString([]rune{97, 98, 99, 252, 162, 8364, 66376, 176, 120})
 }