[DRAFT] make QRE deterministic

[fedimser]

Why?

• QRE, when translating Cirq circuit to trace and encountering classically-controlled gate, does this:

elif isinstance(op, ClassicallyControlledOperation):
    if random.random() < self.classical_control_probability:
        self.handle_op(op.without_classical_controls())

• When we have n IAND gates, they on average produce 0.5*n CZ gates. QRE will not count exactly 0.5*n, but it will count some number close to 0.5*n.
• Let's say I want to optimize trace generation for some gate that I know generates n IAND gates. If I use push_block(n)/IAND/pop_block, instead of getting approximately 0.5*n CZ gates in result, I will get either 0 or n CZ gates, with probability 0.5 each. For large n this introduces large discrepancy, and direction of that discrepancy (over or under estimation) is random.
• My solution is instead of flipping a coin and probabilistically adding non-controlled gate, always add the gate but multiply all counts it produces by 0.5. I.e. replace the code above with:

elif isinstance(op, ClassicallyControlledOperation):
    self.push_block(self.classical_control_probability)
    self.handle_op(op.without_classical_controls())
    self.pop_block()

• This also requires us to change a lot of types in Rust code from u64 to f64.

I am not sure it's the best way to do this, as using real numbers introduces different kind of non-determinism, related to rounding errors. Also clippy doesn't like mixing u64 and f64.