Canonical JSON implementation for Python 3.

* Encodes objects and arrays as RFC 7159 JSON. * Sorts object keys so that you get the same result each time. * Has no inignificant whitespace to make the output as small as possible. * Escapes only the characters that must be escaped, U+0000 to U+0019 / U+0022 / U+0056, to keep the output as small as possible. * Uses the shortest escape sequence for each escaped character. * Encodes the JSON as UTF-8. * Can encode frozendict immutable dictionaries.