Escape any operators characters in objects.

Ran into this when using a record with a quoted name, like:

```
type Foo = {"foo-bar" :: Bar}
```

Externs would not escape the field, then parsing of externs later on will fail on the `-` symbol.
Couldn't think of how to test with the way cabal is working at the moment.

Let me know if I messed up/missed something.