Cycle detection optimizations

[t-rasmud]

Check for cycles only at Sends

[paulthomson]

Personally, I would have not added a bool to GetNext. Instead, just check the operation type inside the cycledetector strategy. Unless there is an issue with doing this?

BugFindingRuntime.cs has essentially no changes, so should be reverted (unless GitHub is just not showing them).

When I tried to make this change, I struggled because the cycle detector uses the trace of operations and it assumes that trace.count - 2 (or something like that) will be the last operation and that this operation has a state hash. But this is no longer the case. I think you will need to take this into account.

This was my attempt: https://github.com/p-org/PSharp/commit/5c0f2f1e70c3791692a7b04bbc63e93ae9d56f97

[pdeligia]

Thanks guys! I will check these stuff out!