Return the keys of dictionaries in order

Using jsonnet with k8s I've noticed an interesting pattern—whenever you specify a list of key-value strings in k8s, jsonnet's objects work better. One way to use that is to map command line aruments, e.g.

```
args_+: {
tls_cert: '/abc',
tls_key: '/def',
}
```

becomes

```--tls-cert=/abc --tls-key=/def```

One problem with that approach is that jsonnet sorts all the keys, thus making it very hard to deal with situations when key order is important.

I'd like to see a mechanism to turn objects into what's basically an OrderedDict of python.