Derive `Arbitrary` for `Sealed<T>` #2061

Job	Run time
miri --no-default-features	5m 10s
features stable	1m 43s
miri	5m 1s
features stable --all-targets	2m 54s
clippy	46s
check no_std	56s
miri --all-features	6m 43s
features nightly	1m 38s
features nightly --all-targets	2m 43s
docs	29s
fmt	11s
test ubuntu-latest stable --no-default-features	1m 1s
wasm	21s
test ubuntu-latest stable	1m 2s
test ubuntu-latest nightly --no-default-features	44s
test ubuntu-latest nightly	52s
test ubuntu-latest 1.79 --no-default-features	31s
test ubuntu-latest 1.79	27s
test windows-latest stable --no-default-features	2m 31s
test windows-latest stable	2m 11s
test windows-latest nightly --no-default-features	2m 9s
test windows-latest nightly	2m 6s
test windows-latest 1.79 --no-default-features	1m 26s
test windows-latest 1.79	1m 23s
test ubuntu-latest 1.79 --features json	29s
test windows-latest 1.79 --features json	1m 26s
test ubuntu-latest nightly --all-features	1m 12s
test windows-latest nightly --all-features	2m 49s
	50m 54s

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