Add 'static bound to waker_ref #2520

Job	Run time
cargo bench	10s
cargo clippy	15s
cargo +1.36 build (futures-{core, io, sink})	58s
cargo +stable build	34s
cargo doc	13s
cargo +beta build	43s
cargo hack check --feature-powerset	13s
cargo fmt	7s
cargo +nightly build	15s
cargo minimal-versions build	13s
cargo miri test	30s
cargo build --target thumbv7m-none-eabi	12s
cargo test -Z sanitizer=address	33s
cargo build --target thumbv6m-none-eabi	25s
cargo test (ubuntu-latest)	16s
cargo test -Z sanitizer=memory	34s
cargo +1.56 build	1m 35s
cargo test (macos-latest)	36s
cargo test -Z sanitizer=thread	33s
cargo test (windows-latest)	27s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	36s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	32s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	39s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	27s
	11m 36s

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