Add sexp decorator

Add a `sexp` optional decorator to `cenum` to output the values as s-expressions.
This is compatible with the `sexplib` convention.  The syntax is;

```
cenum foo64 {
  ONE64;
  TWO64;
  THREE64
} as uint64_t(sexp)
```

And `sexp_of_foo64` and `foo64_of_sexp` functions will also be available.
The representation of the Sexp is the string representation of the enum.