[feature request] Add marker signals to generated traces to help debugging

[tudortimi]

It would make it much easier to debug traces if there were some way of seeing when a property started matching and when it failed. This could be done by adding some dummy signals which would pulse at these times.

[cliffordwolf]

For SVA properties, the <sequence>.triggered mechanism can be used to get the information in which cycle a match ended. There can't be a signal for indicating the cycle in which a match starts, because in that cycle we don't know yet if the property ends up matching or not.