Implement `select`. #4307

Job	Run time
Check nightly-only targets (nightly)	23s
Check --no-default-features (nightly)	53s
Check selected Tier 3 platforms (nightly)	4m 46s
Check (stable)	6m 2s
Rustfmt	4s
Check (nightly)	6m 15s
Check (1.63)	9m 39s
Test rustix_use_experimental_asm (powerpc64le-linux)	47s
Test (ubuntu)	42s
Test use-libc (ubuntu)	50s
Test (ubuntu-20.04)	42s
Test use-libc (i686-linux)	55s
Test use-libc (aarch64-linux)	49s
Test (i686-linux)	53s
Test use-libc (powerpc64le-linux)	47s
Test (aarch64-linux)	46s
Test use-libc (riscv64-linux)	39s
Test (powerpc64le-linux)	50s
Test use-libc (arm-linux)	44s
Test (riscv64-linux)	53s
Test (s390x-linux)	51s
Test (arm-linux)	52s
Test (ubuntu-stable)	37s
Test (i686-linux-stable)	53s
Test (aarch64-linux-stable)	43s
Test (riscv64-linux-stable)	46s
Test (s390x-linux-stable)	52s
Test (powerpc64le-linux-stable)	43s
Test (arm-linux-stable)	40s
Test (ubuntu-1.63)	42s
Test (i686-linux-1.63)	43s
Test (aarch64-linux-1.63)	54s
Test (riscv64-linux-1.63)	52s
Test (s390x-linux-1.63)	51s
Test (powerpc64le-linux-1.63)	53s
Test (arm-linux-1.63)	54s
Test (macos-latest)	42s
Test (macos-12)	34s
Test (windows)	38s
Test (windows-2019)	39s
Test (musl)	36s
	54m 14s

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