TaskAttributed::set_* calls return *this, for chaining #1941

Job	Run time
macos-latest: clang++ Release Threads	12m 46s
macos-latest: clang++ Release OneTBB	3m 29s
macos-latest: clang++ Release PaRSEC	2m 37s
ubuntu-20.04: /usr/bin/g++-9 Release Threads	36m 51s
ubuntu-20.04: /usr/bin/g++-9 Release OneTBB	3m 10s
ubuntu-20.04: /usr/bin/g++-9 Release PaRSEC	1m 59s
macos-latest: clang++ Debug Threads	20m 3s
macos-latest: clang++ Debug OneTBB	2m 53s
macos-latest: clang++ Debug PaRSEC	2m 59s
ubuntu-20.04: /usr/bin/g++-9 Debug Threads	47m 42s
ubuntu-20.04: /usr/bin/g++-9 Debug OneTBB	3m 4s
ubuntu-20.04: /usr/bin/g++-9 Debug PaRSEC	2m 32s
	2h 20m 5s

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