using toStringC14N for stringification is really slow

toStringC14N is really slow and should not be used by default. Ex. with 3.5M XML file:

```
$ time perl -MXML::Chain -lE 'xc(["big-3m.xml"])->as_xml_libxml->toStringC14N'

real	5m8,137s
user	5m8,068s
sys	0m0,056s

$ time perl -MXML::Chain -lE 'xc(["/tmp/big-3m.xml"])->as_xml_libxml->toString'

real	0m0,294s
user	0m0,262s
sys	0m0,033s
```