Use qsim for repeated sampling

[95-martin-orion]

Highlighted in #502.

_sample_measure_results currently calls Cirq for repeated sampling of non-noisy circuits. This is a serviceable solution, but is somewhat less performant than simulating and sampling all on the qsim side. This is magnified by #502, where noiseless circuits on more than 32 qubits are forced to follow the repeated-simulation path.

We should replace the Cirq-dependent behavior with a method that handles this entirely in qsim.