accept refs for the sort datatype accessor

[sameer]

this makes re-using a particular sort with accessors easier, since it
can just be referenced in the builder:

(psuedocode)

let some_complex_sort = Sort::set(&ctx, &Sort::set(...));
let datatype_using_that_sort = DatatypeBuilder::new(&ctx, "foo")
.variant("first", vec![("bar", &some_complex_sort)])
.variant("second", vec![("bar", &some_complex_sort), ("count", &Sort::int(&ctx))])
.finish();

It is backwards incompatible -- users of the builder would need to add a reference. Also, if the builder is not finish-ed immediately, the sort needs a let binding.

[fitzgen]

Thanks!

[fitzgen]

Do you have any other updates like this incoming that I should wait for, or should I cut a release now?

[sameer]

Sorry for the delay, I missed seeing the email notification for this. Looks like there are a few other changes on the way from others, nothing from me though. I can help out and review if needed.