encoding: add option to escape non-ascii characters in JSON This is necessary for hgweb to embed JSON data in HTML. JSON data must be able to be embedded in non-UTF-8 HTML page so long as the page encoding is compatible with ASCII. According to RFC 7159, non-BMP character is represented as UTF-16 surrogate pair. This function first splits an input string into an array of UTF-16 code points. https://tools.ietf.org/html/rfc7159.html#section-7