Introduce StgRetBCO struct in RTS

Motivation

It's a bit hard to grasp how RET_BCO frames are layed out on stack.

This example from Scav.c shows how RET_BCO frames are deconstructed:

    case RET_BCO: {
        StgBCO *bco;
        StgWord size;

        p++;
        evacuate((StgClosure **)p);
        bco = (StgBCO *)*p;
        p++;
        size = BCO_BITMAP_SIZE(bco);
        scavenge_large_bitmap(p, BCO_BITMAP(bco), size);
...
    }

Following the example above, the layout is:

• There is a StgClosure
• Its payload (word after header) is a StgBCO*
• Then, the arguments follow

Proposal

To make this a bit more obvious, introduce struct StgRetBCO:

struct StgRetBCO {
  StgHeader header;
  StgBCO** bco;
  StgWord args[];
}

I haven't tried this, yet. I'm writing this ticket to keep the idea. It may be, that I'm missing something.