Implement AsFd for Unix pipe Sender #501

Job	Run time
TestPoll	38s
MinimalVersions (ubuntu-latest)	19s
MinimalVersions (macos-latest)	29s
Test (ubuntu-latest)	49s
MinimalVersions (windows-latest)	46s
Test (macos-latest)	50s
Test (windows-latest)	2m 1s
TestWakerPipe	36s
CheckTargets (aarch64-apple-darwin)	31s
Clippy	13s
CheckTargets (aarch64-apple-ios)	32s
CheckTargets (aarch64-apple-tvos)	31s
Docs	12s
CheckTargets (aarch64-apple-visionos)	32s
MSRV (ubuntu-latest)	11s
CheckTargets (aarch64-linux-android)	32s
MSRV (macos-latest)	18s
Nightly	21s
CheckTargets (aarch64-unknown-openbsd)	31s
MSRV (windows-latest)	44s
CheckTargets (arm-linux-androideabi)	30s
Rustfmt	13s
CheckTargets (arm64_32-apple-watchos)	32s
Sanitizer (address)	42s
CheckTargets (armv7-sony-vita-newlibeabihf)	27s
Sanitizer (leak)	38s
CheckTargets (i686-unknown-linux-gnu)	30s
Sanitizer (memory)	1m 7s
CheckTargets (sparcv9-sun-solaris)	31s
Sanitizer (thread)	41s
CheckTargets (wasm32-wasi)	31s
CheckTargets (x86_64-apple-darwin)	36s
CheckTargets (x86_64-apple-ios)	34s
CheckTargets (x86_64-pc-solaris)	29s
CheckTargets (x86_64-pc-windows-msvc)	28s
CheckTargets (x86_64-unknown-dragonfly)	29s
CheckTargets (x86_64-unknown-freebsd)	29s
CheckTargets (x86_64-unknown-illumos)	29s
CheckTargets (x86_64-unknown-linux-gnu)	30s
CheckTargets (x86_64-unknown-linux-musl)	29s
CheckTargets (x86_64-unknown-netbsd)	33s
CheckTargets (x86_64-unknown-openbsd)	30s
CheckTargets (x86_64-unknown-redox)	30s
Passed	1s
	24m 5s

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