Add tests for `AsRef`/`AsMut` impls #397

Job	Run time
clippy	21s
test (stable)	45s
careful	59s
rustfmt	13s
build (1.85.0, thumbv7em-none-eabi)	1m 18s
test (1.85.0)	49s
doc	23s
coverage	31s
build (1.85.0, wasm32-unknown-unknown)	1m 24s
miri (x86_64-unknown-linux-gnu)	49s
miri (s390x-unknown-linux-gnu)	1m 6s
build (1.85.0, armv7a-none-eabi)	1m 20s
build (stable, armv7a-none-eabi)	1m 12s
build (stable, thumbv7em-none-eabi)	1m 22s
build (stable, wasm32-unknown-unknown)	1m 11s
	13m 43s

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