Implement `select`. #4313

Job	Run time
Check nightly-only targets (nightly)	22s
Check (stable)	6m 23s
Check (nightly)	6m 27s
Check (1.63)	10m 7s
Rustfmt	7s
Check selected Tier 3 platforms (nightly)	4m 48s
Check --no-default-features (nightly)	53s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 8s
Test (ubuntu)	42s
Test use-libc (ubuntu)	38s
Test (ubuntu-20.04)	43s
Test use-libc (i686-linux)	53s
Test (i686-linux)	56s
Test use-libc (aarch64-linux)	2m 17s
Test use-libc (powerpc64le-linux)	1m 9s
Test (aarch64-linux)	56s
Test use-libc (riscv64-linux)	2m 9s
Test (powerpc64le-linux)	57s
Test use-libc (arm-linux)	2m 1s
Test (riscv64-linux)	55s
Test (s390x-linux)	56s
Test (arm-linux)	56s
Test (ubuntu-stable)	38s
Test (i686-linux-stable)	56s
Test (aarch64-linux-stable)	57s
Test (riscv64-linux-stable)	56s
Test (s390x-linux-stable)	56s
Test (powerpc64le-linux-stable)	55s
Test (arm-linux-stable)	57s
Test (ubuntu-1.63)	44s
Test (i686-linux-1.63)	57s
Test (aarch64-linux-1.63)	56s
Test (riscv64-linux-1.63)	54s
Test (s390x-linux-1.63)	56s
Test (powerpc64le-linux-1.63)	56s
Test (arm-linux-1.63)	57s
Test (macos-latest)	36s
Test (macos-12)	36s
Test (windows)	41s
Test (windows-2019)	43s
Test (musl)	38s
	1h 2m 12s

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