Add open syntax and semantics

This issue will track the progress of adding `open` and `let open` forms to the CakeML syntax and semantics.

It is _almolst_ possible to emulate the functionality of `open` using existing features, but there's no substitute to the re-binding of constructors that is performed by `open`:
```sml
structure S = struct
datatype foo = Foo
end;

fun foo S.Foo = ();

(* This is the only way to lift S.Foo into the top-level namespace: *)
open S
fun foo Foo = ();
```

Some progress has been made on the [dopen](https://github.com/CakeML/cakeml/tree/dopen) branch, and I think the main issue is that the type system is in the way somehow. @xrchz was the last to work on this branch: do you recall what the issues were?