Implement `select`. #4239

Job	Run time
Check --no-default-features (nightly)	54s
Check nightly-only targets (nightly)	31s
Check selected Tier 3 platforms (nightly)	4m 57s
Check (stable)	5m 58s
Rustfmt	7s
Check (nightly)	6m 18s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 4s
Check (1.63)	10m 19s
Test (ubuntu)	42s
Test use-libc (ubuntu)	39s
Test (ubuntu-20.04)	42s
Test use-libc (i686-linux)	1m 11s
Test (i686-linux)	1m 14s
Test use-libc (aarch64-linux)	1m 0s
Test (aarch64-linux)	1m 13s
Test use-libc (powerpc64le-linux)	1m 20s
Test use-libc (riscv64-linux)	59s
Test use-libc (arm-linux)	59s
Test (powerpc64le-linux)	1m 3s
Test (riscv64-linux)	1m 20s
Test (s390x-linux)	1m 10s
Test (arm-linux)	1m 19s
Test (ubuntu-stable)	44s
Test (i686-linux-stable)	1m 13s
Test (aarch64-linux-stable)	1m 2s
Test (riscv64-linux-stable)	1m 3s
Test (s390x-linux-stable)	1m 4s
Test (powerpc64le-linux-stable)	1m 12s
Test (arm-linux-stable)	1m 20s
Test (ubuntu-1.63)	2m 32s
Test (i686-linux-1.63)	2m 42s
Test (aarch64-linux-1.63)	2m 58s
Test (riscv64-linux-1.63)	2m 50s
Test (s390x-linux-1.63)	2m 51s
Test (powerpc64le-linux-1.63)	2m 50s
Test (arm-linux-1.63)	3m 19s
Test (macos-latest)	17m 31s
Test (macos-12)	18m 27s
Test (windows)	52s
Test (windows-2019)	52s
Test (musl)	38s
	1h 50m 59s

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