Improve speed of selected Cirq transformers

[pavoljuhas]

Summarize the task

The following transformers would benefit from performance optimization for larger-size circuits:

• cirq.transformers.gauge_compiling.CZGaugeTransformer
• cirq.merge_single_qubit_moments_to_phxz
• cirq.add_dynamical_decoupling
• cirq.transformers.gauge_compiling.CZGaugeTransformer.as_sweep
• cirq.merge_single_qubit_gates_to_phxz_symbolized

Acceptance criteria – when will the task be considered done?

Each of these transformers is covered by pytest-benchmark and has a PR with significant performance improvement.

Mirroring b/452757715

[RahilJain1366]

Hi @pavoljuhas , can I pick it up if the issue is still open?

[pavoljuhas]

Hi @RahilJain1366 - thank you for your interest. I'd like to take a first stab on this myself, I will let you know if I need help later.