Add methods for reading into unitialized buffers #3621

Job	Run time
Check (stable)	5m 59s
Check (nightly)	5m 57s
Check --no-default-features (nightly)	54s
Check nightly-only targets (nightly)	24s
Check selected Tier 3 platforms (nightly)	4m 10s
Rustfmt	13s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 25s
Check (1.63)	8m 28s
Test (ubuntu)	46s
Test use-libc (ubuntu)	49s
Test (ubuntu-20.04)	44s
Test use-libc (i686-linux)	59s
Test (i686-linux)	56s
Test use-libc (aarch64-linux)	1m 12s
Test (aarch64-linux)	1m 8s
Test use-libc (powerpc64le-linux)	1m 12s
Test (powerpc64le-linux)	1m 7s
Test use-libc (riscv64-linux)	1m 14s
Test (riscv64-linux)	1m 7s
Test use-libc (arm-linux)	1m 10s
Test (s390x-linux)	1m 4s
Test (arm-linux)	1m 3s
Test (ubuntu-stable)	47s
Test (i686-linux-stable)	1m 3s
Test (aarch64-linux-stable)	1m 7s
Test (riscv64-linux-stable)	1m 3s
Test (s390x-linux-stable)	1m 32s
Test (powerpc64le-linux-stable)	1m 18s
Test (arm-linux-stable)	1m 10s
Test (ubuntu-1.63)	4m 33s
Test (i686-linux-1.63)	4m 49s
Test (aarch64-linux-1.63)	4m 53s
Test (riscv64-linux-1.63)	5m 11s
Test (s390x-linux-1.63)	5m 24s
Test (powerpc64le-linux-1.63)	5m 7s
Test (arm-linux-1.63)	4m 55s
Test (macos-latest)	19m 26s
Test (macos-11)	1m 9s
Test (windows)	48s
Test (windows-2019)	53s
Test (musl)	47s
	1h 47m 56s

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