add boundary tests for int64_t
add boundary tests for int64_t

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