refactor!: use `IoBuf` instead of `IoBufMut` in `AsyncWriteRentAt` #680

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	4m 34s
test (x86_64-unknown-linux-gnu, nightly)	4m 17s
test (aarch64-unknown-linux-gnu, stable)	1m 47s
test (aarch64-unknown-linux-gnu, nightly)	4m 29s
test (armv7-unknown-linux-gnueabihf, stable)	1m 45s
test (armv7-unknown-linux-gnueabihf, nightly)	1m 52s
test (riscv64gc-unknown-linux-gnu, stable)	1m 58s
test (riscv64gc-unknown-linux-gnu, nightly)	1m 47s
test (s390x-unknown-linux-gnu, stable)	1m 48s
test (s390x-unknown-linux-gnu, nightly)	1m 58s
test (x86_64-apple-darwin, stable)	1m 10s
test (x86_64-apple-darwin, nightly)	1m 16s
test (aarch64-apple-darwin, stable)	2m 41s
test (aarch64-apple-darwin, nightly)	2m 44s
test (x86_64-pc-windows-gnu, stable)	2m 17s
test (x86_64-pc-windows-gnu, nightly)	2m 30s
test (x86_64-pc-windows-msvc, stable)	4m 14s
test (x86_64-pc-windows-msvc, nightly)	4m 22s
test (i686-pc-windows-gnu, stable)	6m 13s
test (i686-pc-windows-gnu, nightly)	6m 23s
test (i686-pc-windows-msvc, stable)	2m 28s
test (i686-pc-windows-msvc, nightly)	2m 10s
	1h 4m 43s

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