Add `fctrl::SealFlag::F_SEAL_FUTURE_WRITE` #213

Job	Run time
macos	1m 43s
linux_native_builds (x86_64-unknown-linux-gnu)	1m 21s
minver	11s
linux_native_builds (x86_64-unknown-linux-musl)	1m 10s
rustfmt	11s
tier3 (x86_64-unknown-dragonfly)	53s
rust_stable	1m 2s
tier3 (x86_64-unknown-openbsd)	52s
tier3 (x86_64-unknown-haiku)	52s
redox	39s
cross (arm-unknown-linux-gnueabi)	1m 36s
cross (armv7-unknown-linux-gnueabihf)	1m 37s
cross_compiling (aarch64-linux-android)	31s
cross (i686-unknown-linux-gnu)	1m 34s
cross_compiling (arm-linux-androideabi)	33s
cross (i686-unknown-linux-musl)	1m 43s
cross_compiling (armv7-linux-androideabi)	30s
cross (mips-unknown-linux-gnu)	1m 38s
cross_compiling (i686-linux-android)	33s
cross (mips64-unknown-linux-gnuabi64)	1m 36s
cross_compiling (x86_64-linux-android)	30s
cross (mips64el-unknown-linux-gnuabi64)	1m 44s
cross_compiling (arm-unknown-linux-musleabi)	30s
cross (mipsel-unknown-linux-gnu)	1m 39s
cross_compiling (x86_64-unknown-fuchsia)	24s
cross (powerpc64le-unknown-linux-gnu)	3m 17s
cross_compiling (x86_64-unknown-illumos)	24s
cross_compiling (aarch64-apple-ios, true)	23s
cross_compiling (powerpc64-unknown-linux-gnu)	33s
cross_compiling (s390x-unknown-linux-gnu)	34s
cross_compiling (x86_64-unknown-linux-gnux32)	33s
cross_compiling (x86_64-unknown-netbsd)	28s
	31m 44s

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