rename to_arc to clone_arc #1283

Job	Run time
Clang Tidy	5s
Clippy	22s
Build+test (clang, stable, ubuntu-20.04)	28s
Clippy nightly (optional)	23s
Build+test (clang, beta, ubuntu-20.04)	26s
Check for documentation errors	29s
ensure-header-updated	35s
Build+test (clang, nightly, ubuntu-20.04)	28s
Format	13s
Build+test (clang, 1.61.0, ubuntu-20.04)	42s
Check minimum versions	36s
Miri	30s
Build+test (gcc, stable, ubuntu-20.04)	12s
Build+test (gcc, beta, ubuntu-20.04)	22s
Windows	1m 47s
Windows CMake, Debug configuration	1m 48s
Build+test (gcc, nightly, ubuntu-20.04)	21s
Windows CMake, Release configuration	2m 11s
Build+test (gcc, 1.61.0, ubuntu-20.04)	30s
Valgrind	27s
Build+test (macos-latest, clang, stable)	36s
	13m 31s

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