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Fix `FillValue::equals_all` and add `FillValue` tests #118

Job	Run time
clippy	1m 13s
fmt	15s
test	2m 8s
build (stable, --features "")	43s
build (stable, --all-features)	1m 42s
build (stable, --no-default-features)	32s
build (1.71, --features "")	1m 21s
build (1.71, --all-features)	1m 59s
build (1.71, --no-default-features)	33s
build (beta, --features "")	45s
build (beta, --all-features)	2m 8s
build (beta, --no-default-features)	53s
build (nightly, --features "")	43s
build (nightly, --all-features)	1m 59s
build (nightly, --no-default-features)	32s
	17m 26s

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