Remove unsafe code from AssertUnmoved #2333

Job	Run time
cargo +1.56 build	2m 33s
cargo test (ubuntu-latest)	4m 56s
cargo +stable build	1m 16s
cargo test (macos-latest)	6m 1s
cargo bench	2m 23s
cargo +beta build	1m 13s
cargo test (windows-latest)	6m 45s
cargo clippy	36s
cargo +nightly build	1m 18s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	5m 37s
codegen	40s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	9m 28s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	11m 40s
cargo +1.36 build (futures-{core, io, sink})	3m 37s
cargo doc	26s
cargo hack check --feature-powerset	4m 19s
cargo fmt	9s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	4m 31s
cargo miri test	13m 41s
cargo minimal-versions build	39s
cargo build --target thumbv7m-none-eabi	42s
cargo test -Z sanitizer=address	4m 6s
cargo build --target thumbv6m-none-eabi	46s
cargo test -Z sanitizer=memory	4m 41s
cargo test -Z sanitizer=thread	3m 38s
	1h 35m 41s

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