CSON.stringify

As of https://github.com/groupon/cson-safe/pull/9 the upstream `cson-safe` offers a `CSON.stringify` that behaves just as `JSON.stringify` does, but with safe/nice CSON defaults:

```
"foo\nbar" => '''
  foo
  bar
'''
```

Like [the native `JSON.stringify`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify), it leaves to the consumer to decide how to encode or handle non-JSON/CSON:able properties like Function objects and similar with a `visitor` function; I adjusted tests to match, but kept the existing exceptions for CSON:ing `undefined` or a plain `Function` root-level object.

Less code to maintain, if you want to avoid duplicating functionality.
Feels like these checks should be removed since nested functions are now ignored, then this would be more consistent with the `JSON.stringify` behavior.