Add reference implementation for parallel_phase feature #3399

Job	Run time
examples_clang-format	8s
copyright_check	5s
codespell	15s
examples_ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	3m 38s
ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	19m 17s
examples_ubuntu-20.04_g++_cxx17_release_preview=ON	3m 50s
ubuntu-20.04_g++_cxx17_release_preview=ON	17m 20s
examples_ubuntu-20.04_g++-10_cxx20_debug_preview=ON	4m 33s
ubuntu-20.04_g++-10_cxx20_debug_preview=ON	14m 20s
examples_macos-12_clang++_cxx14_relwithdebinfo_preview=ON	4m 14s
ubuntu-22.04_g++-11_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	16m 53s
macos-12_clang++_cxx14_relwithdebinfo_preview=ON	14m 25s
macos-13_clang++_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	15m 55s
python_module_test_ubuntu_latest	35s
examples_windows_cl2019_cxx14_relwithdebinfo_preview=ON	5m 43s
windows_cl2019_cxx14_relwithdebinfo_preview=ON	15m 1s
windows_cl2019_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	20m 52s
examples_windows_cl2022_cxx17_relwithdebinfo_preview=OFF	6m 13s
windows_cl2022_cxx17_relwithdebinfo_preview=OFF	15m 31s
documentation	0s
pages	0s
	2h 58m 48s

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