refactor!: use `IoBuf` instead of `IoBufMut` in `AsyncWriteRentAt` #681

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	5m 4s
test (x86_64-unknown-linux-gnu, nightly)	4m 19s
test (aarch64-unknown-linux-gnu, stable)	1m 47s
test (aarch64-unknown-linux-gnu, nightly)	1m 54s
test (armv7-unknown-linux-gnueabihf, stable)	1m 57s
test (armv7-unknown-linux-gnueabihf, nightly)	1m 50s
test (riscv64gc-unknown-linux-gnu, stable)	1m 46s
test (riscv64gc-unknown-linux-gnu, nightly)	1m 46s
test (s390x-unknown-linux-gnu, stable)	1m 49s
test (s390x-unknown-linux-gnu, nightly)	1m 47s
test (x86_64-apple-darwin, stable)	1m 10s
test (x86_64-apple-darwin, nightly)	1m 15s
test (aarch64-apple-darwin, stable)	2m 39s
test (aarch64-apple-darwin, nightly)	2m 52s
test (x86_64-pc-windows-gnu, stable)	2m 15s
test (x86_64-pc-windows-gnu, nightly)	2m 41s
test (x86_64-pc-windows-msvc, stable)	4m 27s
test (x86_64-pc-windows-msvc, nightly)	4m 45s
test (i686-pc-windows-gnu, stable)	4m 59s
test (i686-pc-windows-gnu, nightly)	5m 19s
test (i686-pc-windows-msvc, stable)	2m 38s
test (i686-pc-windows-msvc, nightly)	2m 20s
	1h 1m 19s

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