impl AsyncReadRentAt and AsyncWriteRentAt for File #710

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	6m 18s
test (x86_64-unknown-linux-gnu, nightly)	8m 43s
test (aarch64-unknown-linux-gnu, stable)	2m 36s
test (aarch64-unknown-linux-gnu, nightly)	3m 58s
test (armv7-unknown-linux-gnueabihf, stable)	2m 39s
test (armv7-unknown-linux-gnueabihf, nightly)	4m 8s
test (riscv64gc-unknown-linux-gnu, stable)	2m 28s
test (riscv64gc-unknown-linux-gnu, nightly)	6m 6s
test (s390x-unknown-linux-gnu, stable)	2m 39s
test (s390x-unknown-linux-gnu, nightly)	4m 17s
test (x86_64-apple-darwin, stable)	1m 55s
test (x86_64-apple-darwin, nightly)	4m 48s
test (aarch64-apple-darwin, stable)	4m 19s
test (aarch64-apple-darwin, nightly)	4m 32s
test (x86_64-pc-windows-gnu, stable)	3m 33s
test (x86_64-pc-windows-gnu, nightly)	4m 35s
test (x86_64-pc-windows-msvc, stable)	6m 33s
test (x86_64-pc-windows-msvc, nightly)	6m 52s
test (i686-pc-windows-gnu, stable)	8m 43s
test (i686-pc-windows-gnu, nightly)	11m 38s
test (i686-pc-windows-msvc, stable)	3m 38s
test (i686-pc-windows-msvc, nightly)	6m 44s
	1h 51m 42s

Provide feedback