Implement `select`. #4312

Job	Run time
Check nightly-only targets (nightly)	22s
Rustfmt	5s
Check --no-default-features (nightly)	53s
Check (stable)	5m 57s
Check selected Tier 3 platforms (nightly)	4m 44s
Check (nightly)	6m 19s
Check (1.63)	9m 17s
Test (ubuntu)	43s
Test rustix_use_experimental_asm (powerpc64le-linux)	59s
Test use-libc (ubuntu)	45s
Test (ubuntu-20.04)	45s
Test use-libc (i686-linux)	54s
Test (i686-linux)	57s
Test use-libc (aarch64-linux)	2m 11s
Test (aarch64-linux)	59s
Test (powerpc64le-linux)	57s
Test use-libc (powerpc64le-linux)	1m 2s
Test (riscv64-linux)	52s
Test use-libc (riscv64-linux)	2m 9s
Test (s390x-linux)	56s
Test use-libc (arm-linux)	1m 53s
Test (arm-linux)	58s
Test (ubuntu-stable)	38s
Test (i686-linux-stable)	57s
Test (aarch64-linux-stable)	59s
Test (riscv64-linux-stable)	58s
Test (s390x-linux-stable)	57s
Test (powerpc64le-linux-stable)	57s
Test (arm-linux-stable)	1m 0s
Test (ubuntu-1.63)	45s
Test (i686-linux-1.63)	57s
Test (aarch64-linux-1.63)	57s
Test (riscv64-linux-1.63)	57s
Test (s390x-linux-1.63)	56s
Test (powerpc64le-linux-1.63)	56s
Test (arm-linux-1.63)	57s
Test (macos-latest)	43s
Test (macos-12)	39s
Test (windows)	42s
Test (windows-2019)	44s
Test (musl)	38s
	1h 0m 54s

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