RSSA backend

[Matthew Fluet]

> Another possibility is to simply add POINTER_SIZE to the frontier
> uncoditionally.  Then you don't need extra labels.

Doesn't that screw up walking the from heap in the GC?  Or do we just make
GC_foreachPointerInObject understand that arrays always have an extra
POINTER_SIZE of dead bytes at the end of a non-zero length array?  I guess
that is fairly simple, although it means we're wasting a little bit of
space on arrays.