run halt modes

[perlindgren]

Simple implementation using vizia gui eventing. This will tie the update frequency to frame rate, later we might want to decouple free-running simulation into a thread.

[perlindgren]

Updated the stim_assert example:

cargo run --example probe_stim_assert

Press play or F5 to start free running mode. Simulation will halt on the failing assertion.

Accesses outside of the assertion array just renders an unknown value (not an error). Not sure that is the behavior we want, but can be useful to test like reset and initialization behavior of circuits.

[onsdagens]

LGTM!