Add reference implementation for parallel_phase feature #3410

Job	Run time
examples_clang-format	10s
codespell	14s
copyright_check	8s
examples_ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	3m 53s
examples_ubuntu-20.04_g++_cxx17_release_preview=ON	3m 49s
ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	19m 18s
ubuntu-20.04_g++_cxx17_release_preview=ON	17m 3s
ubuntu-20.04_g++-10_cxx20_debug_preview=ON	14m 21s
ubuntu-22.04_g++-11_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	17m 42s
examples_ubuntu-20.04_g++-10_cxx20_debug_preview=ON	4m 45s
examples_macos-12_clang++_cxx14_relwithdebinfo_preview=ON	55s
macos-12_clang++_cxx14_relwithdebinfo_preview=ON	52s
macos-13_clang++_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	21m 23s
python_module_test_ubuntu_latest	33s
examples_windows_cl2019_cxx14_relwithdebinfo_preview=ON	1m 26s
windows_cl2019_cxx14_relwithdebinfo_preview=ON	1m 48s
examples_windows_cl2022_cxx17_relwithdebinfo_preview=OFF	1m 58s
windows_cl2019_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	20m 28s
windows_cl2022_cxx17_relwithdebinfo_preview=OFF	3m 14s
documentation	51s
pages	0s
	2h 14m 51s

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