Compiling: Organize Circuit Transformers

[balopat]

Problem: Currently there are many different types of optimizers that serve different purposes spread across cirq and cirq.google. Roughly there are optimizers that perform gate compilation (transforming one gate set to another), simplify circuits in local regions (merging gates, pushing gates through each other, dropping gates near identity), restructure the circuit (stratify and synchronize measurements). The goal of this feature is to clarify which general user optimizers belong in cirq core and to establish documentation as to their behavior and interoperability. Potentially also useful here would be recipes of transformers that go naturally together.

Rough requirements:

• Categorize basic circuit transformer types
• Define boundaries for circuit transformers to be in core circuit
• Provide documentation describing uses of these different types and interoperability.

[dstrain115]

@tanujkhattar Can we close this now? If not, what is remaining to be finished for this?

[tanujkhattar]

The only remaining work is to migrate vendor packages to new transformers infrastructure, but since it's also being tracked in #5028, we can go ahead and close this.