Rought idea of GC rework

1. Decouple GC from Memory Allocation (Remove all this Pool/Arena/Whatever)
2. Made GC track GCable. (Without going through Pools to find if we need more memory/iterating headers/etc)
   1. Implement sweep-free GC. (And it will be much easy because GC track GCable already)
3. Break API to make Parrot_gc_mark_PMC_alive to accept PMC**. (And probably rename it into gc_visit_pmc)
4. Implement Compacting GC.
5. Implement Generational GC on top of Compacting.
6. "Optional" rename VTABLE_mark to VTABLE_gc_visit 'cause I want to reuse it in Compacting phase.
   1. Or try to reuse VTABLE_visit