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.
I tried to keep the behavior consistent with how it was before. But forgot about the behavior down in the tree (oops). If you'd be fine with it, I can remove the check.