implement swap(), add tests by Egtra <yusuke.ichinohe@gmail.com>
implement swap(), add tests by Egtra <yusuke.ichinohe@gmail.com>

turenar/picojson,Banchi0123/picojson,Banchi0123/picojson,pfultz2/picojson,turenar/picojson,kazuho/picojson,karya0/picojson,karya0/picojson,kazuho/picojson,pfultz2/picojson