rename to_arc to clone_arc #1285

Job	Run time
Valgrind	1m 19s
Windows CMake, Release configuration	2m 34s
Windows	2m 7s
Build+test (clang, stable, ubuntu-20.04)	57s
Windows CMake, Debug configuration	1m 8s
Build+test (clang, beta, ubuntu-20.04)	41s
Build+test (clang, nightly, ubuntu-20.04)	44s
Clang Tidy	9s
Clippy	19s
Build+test (clang, 1.61.0, ubuntu-20.04)	4m 17s
Build+test (gcc, stable, ubuntu-20.04)	37s
Clippy nightly (optional)	17s
Build+test (gcc, beta, ubuntu-20.04)	36s
Check for documentation errors	18s
Build+test (gcc, nightly, ubuntu-20.04)	36s
ensure-header-updated	1m 5s
Format	18s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 39s
Build+test (macos-latest, clang, stable)	1m 5s
Check minimum versions	38s
Miri	50s
	25m 14s

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