rework `CastPtr`, `CastConstPtr`, `BoxCastPtr`, `ArcCastPtr` #1232

Job	Run time
Clippy nightly (optional)	21s
Clippy	24s
Build+test (clang, stable, ubuntu-20.04)	1m 15s
Clang Tidy	8s
Build+test (clang, beta, ubuntu-20.04)	1m 13s
Check for documentation errors	22s
ensure-header-updated	2m 18s
Build+test (clang, nightly, ubuntu-20.04)	1m 5s
Build+test (clang, 1.61.0, ubuntu-20.04)	5m 39s
Build+test (gcc, stable, ubuntu-20.04)	54s
Format	17s
Check minimum versions	52s
Miri	1m 3s
Build+test (gcc, beta, ubuntu-20.04)	1m 2s
Build+test (gcc, nightly, ubuntu-20.04)	51s
Windows	2m 24s
Build+test (gcc, 1.61.0, ubuntu-20.04)	4m 43s
Windows CMake, Debug configuration	2m 42s
Windows CMake, Release configuration	3m 44s
Build+test (macos-latest, clang, stable)	1m 31s
Valgrind	1m 23s
	34m 11s

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