Use unsafe.string instead of unsafe.pointer

This only a cosmetic change, the speed seems to be exactly the same

msgp/unsafe.go

@@ -24,7 +24,12 @@ const (
 // THIS IS EVIL CODE.
 // YOU HAVE BEEN WARNED.
 func UnsafeString(b []byte) string {
-	return *(*string)(unsafe.Pointer(&b))
+	var p *byte
+	l := len(b)
+	if l != 0 {
+		p = &b[0]
+	}
+	return unsafe.String(p, l)
 }
 
 // UnsafeBytes returns the string as a byte slice