Implement Deref{Mut} for AllowStdIo. #2406

Job	Run time
cargo bench	52s
codegen	25s
cargo +stable build	37s
cargo clippy	22s
cargo +beta build	41s
cargo +1.36 build (futures-{core, io, sink})	2m 1s
cargo +nightly build	43s
cargo doc	19s
cargo hack check --feature-powerset	2m 43s
cargo fmt	5s
cargo miri test	7m 13s
cargo minimal-versions build	29s
cargo build --target thumbv7m-none-eabi	32s
cargo build --target thumbv6m-none-eabi	27s
cargo test -Z sanitizer=address	2m 57s
cargo test (ubuntu-latest)	2m 54s
cargo test -Z sanitizer=memory	3m 36s
cargo +1.56 build	2m 39s
cargo test -Z sanitizer=thread	2m 19s
cargo test (macos-latest)	6m 19s
cargo test (windows-latest)	4m 30s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 31s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 23s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 11s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 7s
	1h 2m 55s

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