feat: add `Default` for `Sealed<T>` (#755) #2073

Job	Run time
clippy	1m 14s
docs	53s
check no_std	32s
features stable	1m 47s
fmt	13s
miri --no-default-features	5m 1s
features stable --all-targets	2m 44s
miri	5m 3s
test ubuntu-latest stable --no-default-features	1m 3s
features nightly	2m 25s
miri --all-features	6m 50s
wasm	26s
test ubuntu-latest stable	56s
features nightly --all-targets	3m 28s
test ubuntu-latest nightly --no-default-features	1m 12s
test ubuntu-latest nightly	1m 18s
test ubuntu-latest 1.79 --no-default-features	34s
test ubuntu-latest 1.79	31s
test windows-latest stable --no-default-features	2m 10s
test windows-latest stable	2m 7s
test windows-latest nightly --no-default-features	2m 37s
test windows-latest nightly	2m 37s
test windows-latest 1.79 --no-default-features	1m 27s
test windows-latest 1.79	1m 24s
test ubuntu-latest 1.79 --features json	33s
test windows-latest 1.79 --features json	1m 9s
test ubuntu-latest nightly --all-features	1m 26s
test windows-latest nightly --all-features	2m 52s
	54m 32s

Provide feedback