Make unique boxes self-describing

[brson]

Unique boxes will need to be self describing so that the unwinder knows what's in them.

[brson]

All things in the exchange heap now have a four word box header like gc boxes, but the type descriptors are not correct.

[thestinger]

These headers aren't going to be needed since only Owned types will go on the exchange heap.

[steveklabnik]

Oh my, I typo'd "2279' instead of "2779".

[huonw]

(Was already closed.)