Support canonicalizing interface pointers.

[zenhack]

This is consistent with what the C++ and Haskell implementations do;
rather than failing, simply copy the index.

---

I need this for something I'm working on where I separately serialize a CapTable to disk, and then store a canoniclized structure like:

struct Stored(T) {
  value @0 :T;
  caps @1 :List(StoredCap);
}

...such that upon reading back the data I can reconstruct the CapTable from the caps field -- but I need to be able to retain interface pointers inside value when canonicalizing.