Merge pull request #22 from avsm/master

Add `to_bigarray` to convert back into a Bigarray slice.
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.

Closes #20
regenerate with oasis 0.4.4
travis: stop testing 3.12.1, and install mirage to check it all works
travis: reduce verbosity
travis: workaround conduit-async unavailability on 4.00.1
travis: clean before pin
Merge pull request #21 from avsm/add-sexp-decorator

Add sexp decorator