Prototype a new `Buffer` trait. #4779

Job	Run time
Rustfmt	13s
Check nightly-only targets (nightly)	24s
Check --no-default-features (nightly)	19s
Check (stable)	6m 29s
Check selected Tier 3 platforms (nightly)	5m 27s
Check (nightly)	6m 36s
Check (1.63)	10m 22s
Test (ubuntu)	44s
Test (ubuntu-20.04)	44s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 9s
Test use-libc (ubuntu)	46s
Test (i686-linux)	1m 10s
Test rustix_use_experimental_asm (s390x-linux)	1m 26s
Test use-libc (i686-linux)	1m 18s
Test (aarch64-linux)	1m 11s
Test use-libc (aarch64-linux)	1m 21s
Test (powerpc64le-linux)	1m 9s
Test (riscv64-linux)	1m 11s
Test use-libc (powerpc64le-linux)	1m 19s
Test use-libc (riscv64-linux)	1m 18s
Test (s390x-linux)	1m 8s
Test use-libc (s390x-linux)	1m 28s
Test (arm-linux)	1m 11s
Test (ubuntu-stable)	45s
Test use-libc (arm-linux)	1m 23s
Test (i686-linux-stable)	1m 7s
Test (aarch64-linux-stable)	1m 12s
Test (riscv64-linux-stable)	1m 9s
Test (s390x-linux-stable)	1m 8s
Test (powerpc64le-linux-stable)	1m 9s
Test (arm-linux-stable)	1m 9s
Test (ubuntu-1.63)	1m 9s
Test (i686-linux-1.63)	1m 10s
Test (aarch64-linux-1.63)	1m 12s
Test (riscv64-linux-1.63)	1m 10s
Test (s390x-linux-1.63)	1m 8s
Test (powerpc64le-linux-1.63)	1m 10s
Test (arm-linux-1.63)	1m 9s
Test (macos-latest)	41s
Test (macos-13)	1m 8s
Test (windows)	51s
Test (windows-2019)	53s
Test (musl)	48s
	1h 9m 54s

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