rename to_arc to clone_arc #1286

Job	Run time
Build+test (clang, stable, ubuntu-20.04)	1m 6s
Build+test (clang, beta, ubuntu-20.04)	37s
Check for documentation errors	16s
Windows	1m 45s
Windows CMake, Debug configuration	2m 2s
Build+test (clang, nightly, ubuntu-20.04)	38s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 11s
Windows CMake, Release configuration	2m 26s
Build+test (gcc, stable, ubuntu-20.04)	35s
Valgrind	51s
Build+test (gcc, beta, ubuntu-20.04)	37s
Build+test (gcc, nightly, ubuntu-20.04)	34s
Clang Tidy	6s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 24s
Build+test (macos-latest, clang, stable)	1m 40s
Clippy	18s
Clippy nightly (optional)	20s
ensure-header-updated	1m 4s
Format	15s
Check minimum versions	38s
Miri	50s
	25m 13s

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