Custom-state-representation simulator infra

[daxfohl]

Infrastructure for third-parties to easily create custom Cirq-compatible simulators.

This PR makes it easy to create custom simulators that support all the advanced Cirq features such as

• subcircuits
• classical controls (sympy conditions, indexed conditions)
• repeat_until loops
• noise models
• param resolvers
• parameterized repetitions
• product-state mode

As an example in the tests, a ComputationalBasisState simulator supporting all the above can be implemented in under 20 LOC.

[daxfohl]

@95-martin-orion wdyt here? Useful, not useful? Now or post-1.0?

[95-martin-orion]

@95-martin-orion wdyt here? Useful, not useful? Now or post-1.0?

Yeah, I think this is potentially interesting pre-1.0. The set of simulators it supports is rather limited [1], but there's a good number of simulators in that space and as the test shows it's quite powerful for that subset. I'll give a more detailed review in a bit.

[1] The stepwise structure rules out tensor-network simulators like quimb, and the tight Cirq coupling (simulation uses act_on methods and requires read/write access to the classical data during simulation) would be difficult to implement in non-Python simulators like qsim without a performance overhead.

[95-martin-orion]

I think contrib is the right place for this so that it's clearly separate from the core simulator API, but it's a nicely encapsulated example for using the simulator internals.