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.

src/runtime/string.go

@@ -89,6 +89,30 @@ func stringToBytes(x _string) (slice struct {
 	return
 }
 
+// Convert a []rune slice to a string.
+func stringFromRunes(runeSlice []rune) (s _string) {
+	// Count the number of characters that will be in the string.
+	for _, r := range runeSlice {
+		_, numBytes := encodeUTF8(r)
+		s.length += numBytes
+	}
+
+	// Allocate memory for the string.
+	s.ptr = (*byte)(alloc(s.length))
+
+	// Encode runes to UTF-8 and store the resulting bytes in the string.
+	index := uintptr(0)
+	for _, r := range runeSlice {
+		array, numBytes := encodeUTF8(r)
+		for _, c := range array[:numBytes] {
+			*(*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(s.ptr)) + index)) = c
+			index++
+		}
+	}
+
+	return
+}
+
 // Convert a string to []rune slice.
 func stringToRunes(s string) []rune {
 	var n = 0