impl AsyncReadRentAt and AsyncWriteRentAt for File #708

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	5m 3s
test (x86_64-unknown-linux-gnu, nightly)	8m 45s
test (aarch64-unknown-linux-gnu, stable)	2m 31s
test (aarch64-unknown-linux-gnu, nightly)	6m 14s
test (armv7-unknown-linux-gnueabihf, stable)	2m 34s
test (armv7-unknown-linux-gnueabihf, nightly)	4m 2s
test (riscv64gc-unknown-linux-gnu, stable)	2m 45s
test (riscv64gc-unknown-linux-gnu, nightly)	4m 6s
test (s390x-unknown-linux-gnu, stable)	2m 26s
test (s390x-unknown-linux-gnu, nightly)	4m 3s
test (x86_64-apple-darwin, stable)	1m 48s
test (x86_64-apple-darwin, nightly)	2m 25s
test (aarch64-apple-darwin, stable)	3m 55s
test (aarch64-apple-darwin, nightly)	6m 45s
test (x86_64-pc-windows-gnu, stable)	3m 41s
test (x86_64-pc-windows-gnu, nightly)	4m 19s
test (x86_64-pc-windows-msvc, stable)	6m 14s
test (x86_64-pc-windows-msvc, nightly)	7m 19s
test (i686-pc-windows-gnu, stable)	8m 32s
test (i686-pc-windows-gnu, nightly)	10m 47s
test (i686-pc-windows-msvc, stable)	3m 7s
test (i686-pc-windows-msvc, nightly)	6m 31s
	1h 47m 52s

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