compacting GC

[mmoskal]

Some notes on how to implement compacting/moving GC:

• [ ] in addition to marking things as PENDING in too deep GC mark, also have a tmp list (16 elements or so)
• [ ] devs_value_pin(value_t) -> devs_add_root(value_t*) or devs_add_root(void**)
• [ ] we'll need separate malloc() area, for pointers that can't move

GC Process:

• mark live pointers as today
• compute final size S of the heap (sum of sizes of live objects)
• rewrite first word of each live object forwarding pointer, while moving the existing header from first word to linked list past S
• rewrite internal pointers and roots using forwarding pointers
• compact the heap, while fixing the first word from forwarding pointer back into header