Add reference implementation for parallel_phase feature #3465

Job	Run time
copyright_check	5s
examples_clang-format	5s
examples_ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	3m 37s
codespell	14s
examples_ubuntu-20.04_g++_cxx17_release_preview=ON	3m 44s
ubuntu-latest_g++_cxx14_relwithdebinfo_preview=OFF	19m 13s
examples_ubuntu-20.04_g++-10_cxx20_debug_preview=ON	4m 42s
examples_macos-15_clang++_cxx14_relwithdebinfo_preview=ON	2m 39s
ubuntu-20.04_g++_cxx17_release_preview=ON	17m 31s
macos-14_clang++_cxx14_relwithdebinfo_preview=ON	9m 0s
ubuntu-20.04_g++-10_cxx20_debug_preview=ON	14m 35s
macos-13_clang++_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	16m 16s
python_module_test_ubuntu_latest	31s
ubuntu-22.04_g++-11_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	17m 42s
examples_windows_cl2019_cxx14_relwithdebinfo_preview=ON	5m 53s
examples_windows_cl2022_cxx17_relwithdebinfo_preview=OFF	5m 48s
windows_cl2019_cxx14_relwithdebinfo_preview=ON	14m 20s
windows_cl2019_cxx20_release_preview=ON-DBUILD_SHARED_LIBS=OFF	21m 33s
windows_cl2022_cxx17_relwithdebinfo_preview=OFF	15m 27s
documentation	49s
pages	0s
	2h 53m 44s

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