FdSet: allow conversion from iterable of AsFd #1000

Job	Run time
linux_native_builds (x86_64-unknown-linux-gnu)	2m 41s
macos	3m 55s
linux_native_builds (x86_64-unknown-linux-musl)	2m 39s
macos-aarch64	2m 22s
minver	12s
rust_stable	1m 17s
rustfmt	9s
tier3 (x86_64-unknown-dragonfly)	59s
tier3 (x86_64-unknown-openbsd)	55s
tier3 (x86_64-unknown-haiku)	53s
redox	36s
cross_compiling (aarch64-linux-android)	34s
cross (arm-unknown-linux-gnueabi)	1m 44s
cross_compiling (arm-linux-androideabi)	29s
cross (armv7-unknown-linux-gnueabihf)	1m 47s
cross_compiling (armv7-linux-androideabi)	36s
cross (i686-unknown-linux-gnu)	1m 41s
cross_compiling (i686-linux-android)	35s
cross (i686-unknown-linux-musl)	1m 42s
cross_compiling (x86_64-linux-android)	31s
cross (mips-unknown-linux-gnu)	1m 39s
cross_compiling (arm-unknown-linux-musleabi)	36s
cross (mips64-unknown-linux-gnuabi64)	1m 42s
cross_compiling (x86_64-unknown-fuchsia)	28s
cross (mips64el-unknown-linux-gnuabi64)	1m 41s
cross_compiling (x86_64-unknown-illumos)	32s
cross (mipsel-unknown-linux-gnu)	1m 45s
cross_compiling (aarch64-apple-ios, true)	30s
cross (powerpc64le-unknown-linux-gnu)	1m 42s
cross_compiling (powerpc64-unknown-linux-gnu)	37s
cross_compiling (s390x-unknown-linux-gnu)	38s
cross_compiling (x86_64-unknown-linux-gnux32)	36s
cross_compiling (x86_64-unknown-netbsd)	28s
	39m 11s

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