Can we haz const new Mutex? #2914

Job	Run time
cargo clippy	16s
cargo bench	49s
cargo +1.36 build (futures-{core, io, sink})	55s
cargo +stable build	32s
cargo +beta build	38s
cargo doc	18s
cargo hack check --feature-powerset	2m 16s
cargo +nightly build	39s
cargo fmt	10s
cargo minimal-versions build	21s
cargo miri test	13m 22s
cargo build --target thumbv7m-none-eabi	26s
cargo test -Z sanitizer=address	3m 15s
cargo build --target thumbv6m-none-eabi	27s
cargo test -Z sanitizer=memory	3m 51s
cargo test (ubuntu-latest)	2m 41s
cargo test -Z sanitizer=thread	2m 27s
cargo +1.56 build	1m 3s
cargo test (macos-latest)	6m 36s
cargo test (windows-latest)	4m 37s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 26s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 40s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 45s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 31s
	1h 6m 1s

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