Prototype a new `Buffer` trait. #4780

Job	Run time
Check nightly-only targets (nightly)	20s
Check (stable)	6m 18s
Rustfmt	13s
Check (nightly)	6m 7s
Check --no-default-features (nightly)	56s
Check selected Tier 3 platforms (nightly)	5m 37s
Check (1.63)	11m 10s
Test use-libc (ubuntu)	48s
Test (ubuntu)	44s
Test use-libc (i686-linux)	1m 20s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 12s
Test (ubuntu-20.04)	45s
Test use-libc (aarch64-linux)	1m 16s
Test rustix_use_experimental_asm (s390x-linux)	1m 13s
Test (i686-linux)	1m 6s
Test use-libc (powerpc64le-linux)	1m 23s
Test (aarch64-linux)	1m 7s
Test use-libc (riscv64-linux)	1m 20s
Test use-libc (s390x-linux)	2m 3s
Test (powerpc64le-linux)	1m 4s
Test use-libc (arm-linux)	1m 27s
Test (riscv64-linux)	1m 4s
Test (s390x-linux)	1m 5s
Test (arm-linux)	1m 5s
Test (ubuntu-stable)	45s
Test (i686-linux-stable)	53s
Test (aarch64-linux-stable)	1m 5s
Test (riscv64-linux-stable)	1m 5s
Test (s390x-linux-stable)	1m 3s
Test (powerpc64le-linux-stable)	1m 3s
Test (arm-linux-stable)	1m 5s
Test (ubuntu-1.63)	1m 3s
Test (i686-linux-1.63)	1m 5s
Test (aarch64-linux-1.63)	1m 5s
Test (riscv64-linux-1.63)	1m 3s
Test (s390x-linux-1.63)	1m 4s
Test (powerpc64le-linux-1.63)	1m 5s
Test (arm-linux-1.63)	1m 6s
Test (macos-latest)	48s
Test (macos-13)	1m 6s
Test (windows)	45s
Test (windows-2019)	51s
Test (musl)	47s
	1h 9m 30s

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