JSON parsing allows non-whitespace trailing characters

Picojson supports a streaming mode in which a stream containing a series of JSON values can be repeatedly parsed. For this reason, it does not return an error when passed a string containing a valid JSON value followed by non-whitespace trailing characters.

However, in addition to the four-argument picojson::parse() that we're using, picojson contains a two-argument parse() function (https://github.com/kazuho/picojson/blob/master/picojson.h#L938-L942) which accepts a std::string and should probably validate its input to ensure it doesn't contain trailing characters. A pull request has been filed for this change at https://github.com/kazuho/picojson/pull/70 and if it's merged, we can switch to the two-argument function call. In the meantime, we should provide such input validation ourselves in JSON::parse().