Deprecate optimize_for_sycamore and optimize_for_xmon

[verult]

• Added deprecation warnings
• Updated tests to check for deprecation warnings.
  @tanujkhattar @MichaelBroughton

[MichaelBroughton]

LGTM, after updating the examples.

[tanujkhattar]

@verult Ideally, we need to create google-specific versions of cz and sqrt-iswap target gatesets, which correctly handle wait gates and have additional postprocess transformers like eject_z and eject_phased_paulis.

It would be good to evaluate whether the proposed fix (i.e. using cirq.optimize_for_target_gateset with cirq-core gatesets) will be end up breaking the users flow who depend upon cg.optimized_for_sycamore or not. One way to evaluate this would be to rewrite the existing tests of cg.optimized_for_sycamore using cirq.optimize_for_target_gateset as well.

[verult]

Sounds good. I'm focusing on SerializableDevice and SerializableGateSet deprecations for the time being (the scope is growing larger than I anticipated). Not sure if I can get to this before the release deadline, but will take a look right after deprecations are done.

[verult]

@MichaelBroughton and I chatted about this and we decided to postpone the evaluation until after the 0.15 release.

If tests break, we could perhaps make a patch release to update the deprecation warning if it's not too time consuming to do.