Add "borrowed-fd" feature for sending BorrowedFd on a socket #52

Job	Run time
bare-cross-build (x86_64-unknown-openbsd)	26s
native-test (nightly, ubuntu-latest, --all-features)	20s
cross-ios-build (nightly, aarch64-apple-ios)	33s
native-test (nightly, ubuntu-latest, --all-features --release)	15s
cross-ios-build (nightly, x86_64-apple-ios)	24s
native-test (nightly, ubuntu-latest)	12s
cross-ios-build (stable, aarch64-apple-ios)	23s
native-test (nightly, macOS-latest, --all-features)	28s
cross-ios-build (stable, x86_64-apple-ios)	21s
native-test (nightly, macOS-latest, --all-features --release)	28s
native-test (nightly, macOS-latest)	21s
native-test (stable, ubuntu-latest, --all-features)	10s
native-test (stable, ubuntu-latest, --all-features --release)	14s
native-test (stable, ubuntu-latest)	16s
native-test (stable, macOS-latest, --all-features)	14s
native-test (stable, macOS-latest, --all-features --release)	28s
native-test (stable, macOS-latest)	23s
native-test (1.53.0, ubuntu-latest, --all-features)	2m 24s
native-test (1.53.0, ubuntu-latest, --all-features --release)	2m 22s
native-test (1.53.0, ubuntu-latest)	2m 22s
native-test (1.53.0, macOS-latest, --all-features)	3m 48s
native-test (1.53.0, macOS-latest, --all-features --release)	4m 6s
native-test (1.53.0, macOS-latest)	4m 22s
	25m 20s

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