feat: impl AsFd for PtyMaster #910

Job	Run time
minver	13s
macos-aarch64	1m 50s
macos	3m 38s
linux_native_builds (x86_64-unknown-linux-gnu)	2m 35s
linux_native_builds (x86_64-unknown-linux-musl)	2m 25s
rust_stable	1m 17s
rustfmt	5s
tier3 (x86_64-unknown-openbsd)	55s
redox	39s
cross (arm-unknown-linux-gnueabi)	1m 45s
cross_compiling (aarch64-linux-android)	32s
cross (armv7-unknown-linux-gnueabihf)	1m 41s
cross_compiling (arm-linux-androideabi)	35s
cross (i686-unknown-linux-gnu)	1m 42s
cross_compiling (armv7-linux-androideabi)	33s
cross (i686-unknown-linux-musl)	1m 58s
cross_compiling (i686-linux-android)	36s
cross (mips-unknown-linux-gnu)	1m 42s
cross_compiling (x86_64-linux-android)	31s
cross (mips64-unknown-linux-gnuabi64)	1m 50s
cross (mips64el-unknown-linux-gnuabi64)	1m 52s
cross_compiling (arm-unknown-linux-musleabi)	33s
cross (mipsel-unknown-linux-gnu)	1m 44s
cross_compiling (x86_64-unknown-fuchsia)	30s
cross (powerpc64le-unknown-linux-gnu)	1m 40s
cross_compiling (x86_64-unknown-illumos)	25s
cross_compiling (aarch64-apple-ios, true)	30s
cross_compiling (powerpc64-unknown-linux-gnu)	36s
cross_compiling (s390x-unknown-linux-gnu)	33s
cross_compiling (x86_64-unknown-linux-gnux32)	39s
cross_compiling (x86_64-unknown-netbsd)	29s
	36m 33s

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