Interpreter interruptions

There are multiple reasons for which the interpreter execution may need to be interrupted and control returned:
- Time slicing when running multiple non-cooperative applets. Applets have a time quota and the scheduler uses a hardware timer to switch applets when the quota is reached. Quota replenishes according to the scheduling strategy (round-robin, priorities, etc).
- Bounded interrupt latency. When an interrupt occurs and an applet has a handler registered, the interpreter needs to be paused (even outside a host function call) to execute that handler.

Design questions/ideas:
- There could be either a global or per-store atomic boolean set to true when the interpreter should return control.
- The interpreter should check this boolean often enough for bounded latency but not too often for performance. An option would be to check on every non-forward edge (i.e. branching to the beginning of a loop or calling a function).