Add `OwnedMappedMutexGuard` for `OwnedMutexGuard` #2651

Job	Run time
cargo bench	19s
cargo clippy	21s
cargo +stable build	39s
cargo +1.36 build (futures-{core, io, sink})	55s
cargo +beta build	45s
cargo doc	18s
cargo +nightly build	24s
cargo hack check --feature-powerset	2m 16s
cargo fmt	11s
cargo minimal-versions build	29s
cargo miri test	46s
cargo test -Z sanitizer=address	39s
cargo build --target thumbv7m-none-eabi	19s
cargo test (ubuntu-latest)	22s
cargo build --target thumbv6m-none-eabi	17s
cargo test -Z sanitizer=memory	42s
cargo test (macos-latest)	25s
cargo +1.56 build	1m 19s
cargo test (windows-latest)	53s
cargo test -Z sanitizer=thread	53s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	57s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	40s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	40s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	41s
	16m 10s

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