Multi-cap support! Maybe it works.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Reorganize to store gc_thread in RCs

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
GC partial

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Everything except Evac and Scav

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Cutover.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>