We should make it impossible for any lisp object to have an address below #x01000000 (16M).
If we guarantee this, we can make it legal for node registers to contain unboxed values below
2^24 (which, conveniently enough, is ARRAY-TOTAL-SIZE-LIMIT: we could therefore put unboxed
vector indexes into node registers.)
Certainly the lisp heap would have to start at #x01000000 or above, and stacks probably would too.
If we can do this, then the GC will still be able to reliably distinguish nodes from non-nodes, and we'll
be able to avoid doing mark-as-imm/mark-as-node in many cases.
