rename to_arc to clone_arc #1287

Job	Run time
Clang Tidy	4s
Clippy nightly (optional)	18s
Clippy	19s
Build+test (clang, stable, ubuntu-20.04)	39s
Build+test (clang, beta, ubuntu-20.04)	38s
Check for documentation errors	15s
ensure-header-updated	1m 2s
Format	13s
Build+test (clang, nightly, ubuntu-20.04)	38s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 25s
Check minimum versions	35s
Windows	1m 51s
Build+test (gcc, stable, ubuntu-20.04)	34s
Miri	47s
Windows CMake, Debug configuration	1m 15s
Build+test (gcc, beta, ubuntu-20.04)	36s
Windows CMake, Release configuration	1m 40s
Build+test (gcc, nightly, ubuntu-20.04)	35s
Valgrind	42s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 20s
Build+test (macos-latest, clang, stable)	1m 14s
	22m 40s

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