Implement `select`. #4309

Job	Run time
Check nightly-only targets (nightly)	22s
Check (stable)	6m 25s
Check selected Tier 3 platforms (nightly)	4m 52s
Check (nightly)	6m 7s
Check --no-default-features (nightly)	55s
Check (1.63)	9m 28s
Rustfmt	4s
Test rustix_use_experimental_asm (powerpc64le-linux)	2m 16s
Test (ubuntu)	43s
Test use-libc (ubuntu)	39s
Test (ubuntu-20.04)	42s
Test (i686-linux)	1m 0s
Test use-libc (i686-linux)	1m 10s
Test (aarch64-linux)	1m 4s
Test use-libc (aarch64-linux)	2m 23s
Test (powerpc64le-linux)	1m 3s
Test use-libc (powerpc64le-linux)	2m 13s
Test (riscv64-linux)	1m 3s
Test use-libc (riscv64-linux)	2m 20s
Test (s390x-linux)	1m 3s
Test use-libc (arm-linux)	2m 5s
Test (arm-linux)	1m 8s
Test (ubuntu-stable)	38s
Test (i686-linux-stable)	58s
Test (aarch64-linux-stable)	1m 4s
Test (riscv64-linux-stable)	1m 6s
Test (s390x-linux-stable)	1m 4s
Test (powerpc64le-linux-stable)	1m 5s
Test (arm-linux-stable)	1m 5s
Test (ubuntu-1.63)	1m 2s
Test (i686-linux-1.63)	1m 3s
Test (aarch64-linux-1.63)	1m 6s
Test (riscv64-linux-1.63)	1m 4s
Test (s390x-linux-1.63)	1m 3s
Test (powerpc64le-linux-1.63)	1m 4s
Test (arm-linux-1.63)	1m 5s
Test (macos-latest)	28s
Test (macos-12)	47s
Test (windows)	49s
Test (windows-2019)	50s
Test (musl)	38s
	1h 7m 4s

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