[simulator] Try speeding up the simulator

[Phionx]

There are several potential ways to speed up benchmarking simulations.

• Currently, we are inserting identity gates where we want errors, which forces us to use an optimization level of 0 thus preventing additional optimizations in the rest of the circuit. Instead, we can insert the Krauss error operators directly.
• It may be faster to use density matrix simulation and then use that to measure the average value of the desired measurement operator (say logical Z readout) as follows, Tr(rho * measurement_operator).
• We should look into why adding single qubit gates on the same qubit drastically increases simulation time. This may be because circuit depth is increasing with each addition gate on the same qubit.