Bring GateTabulation out of cirq-google to cirq-core

[Ashalynd]

As discussed with @tanujkhattar in #4362, creating a separate issue to track GateTabulation refactoring.

[tanujkhattar]

@Ashalynd As part of tracking the progress towards our Cirq 1.0 roadmap items, I wanted to check in on the progress on this issue. Could you start working on it yet?

[Ashalynd]

Hi Tanuj, I had to put it on hold for a few weeks due to 80% work. Expect to create a PR this week.

[Ashalynd]

Hey, a quick update: started working on it, fighting with a couple tests :)

[Ashalynd]

Question: does it make sense to also move the code that is using GateTabulation now?

[tanujkhattar]

Optimizers like ConvertToSycamoreGates should stay in cirq_google and use cirq.GateTabulation object. So no, the google specific code using GateTabulation shouldn't be moved to cirq-core.