Simplify FrameMap: make offsets of spill slots and outgoing parameters independent so that they can be allocated at the same time, eliminating the separate phases. This makes the separate StackBlock unnecesary. Change CiStackSlot to use byte offsets instead of spill slot index. This makes CiTarget.spillSlotSize unnecessary.

// I can cast any old thing I want to this type object:
public class You_Have_Been_P0wned {
  // Make a bunch of int-fields so I can peek & poke it
  int _a;
  int _b;
  int _c;
  int _d;

}