Implement `select`. #4306

Job	Run time
Check (stable)	6m 14s
Check (nightly)	6m 10s
Check --no-default-features (nightly)	57s
Check nightly-only targets (nightly)	22s
Check (1.63)	9m 20s
Check selected Tier 3 platforms (nightly)	4m 52s
Rustfmt	6s
Test rustix_use_experimental_asm (powerpc64le-linux)	54s
Test use-libc (ubuntu)	24s
Test use-libc (i686-linux)	41s
Test use-libc (aarch64-linux)	41s
Test (ubuntu)	30s
Test use-libc (powerpc64le-linux)	41s
Test (ubuntu-20.04)	33s
Test use-libc (riscv64-linux)	41s
Test (i686-linux)	43s
Test use-libc (arm-linux)	39s
Test (aarch64-linux)	44s
Test (powerpc64le-linux)	42s
Test (riscv64-linux)	35s
Test (s390x-linux)	43s
Test (arm-linux)	33s
Test (ubuntu-stable)	24s
Test (i686-linux-stable)	43s
Test (aarch64-linux-stable)	39s
Test (riscv64-linux-stable)	39s
Test (s390x-linux-stable)	40s
Test (powerpc64le-linux-stable)	40s
Test (arm-linux-stable)	44s
Test (ubuntu-1.63)	42s
Test (i686-linux-1.63)	44s
Test (aarch64-linux-1.63)	43s
Test (riscv64-linux-1.63)	31s
Test (s390x-linux-1.63)	43s
Test (powerpc64le-linux-1.63)	44s
Test (arm-linux-1.63)	36s
Test (macos-latest)	27s
Test (macos-12)	42s
Test (windows)	29s
Test (windows-2019)	31s
Test (musl)	27s
	49m 33s

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