Lisp-style signals

[Mercerenies]

We have a nice exception system, and it could probably be extended to support this.

When an exception is thrown, the handlers are queued onto the call stack, followed by a termination instruction. If we don't queue the termination instruction, we could "handle" an exception and then resume code.

The main issue is preserving the general-purpose registers (and possibly some other register info) when we do this.

[Mercerenies]

This may be complicated by the IntState / TransientState split in 717f33d