- Create a new kind of stack slot: emergency_slot.
- Add a get_emergency_slot() method which finds a suitable emergency
slot given a list of slots already in use.
- Use emergency spill slots when schedule_moves needs them.
This is trying to keep algorithms out if the ir module which deals with
the intermediate representation.
Also give the layout_stack() function a Result return value so it can
report a soft error when the stack frame is too large instead of
asserting. Since local variables can be arbitrarily large, it is easy
enough to overflow the stack with even a small function.
