Add `Level` type #1725

Job	Run time
check	1m 9s
fuzz record_ref	6m 36s
fuzz select_query	7m 53s
cross x86_64-unknown-linux-gnu (stable)	1m 35s
cross x86_64-unknown-linux-gnu (beta)	2m 5s
cross x86_64-unknown-linux-gnu (nightly)	2m 9s
cross x86_64-unknown-linux-musl (stable)	1m 53s
cross aarch64-unknown-linux-gnu (stable)	1m 34s
cross aarch64-unknown-linux-musl (stable)	2m 4s
test x86_64-unknown-linux-gnu (stable)	2m 13s
test x86_64-unknown-linux-gnu (beta)	2m 8s
test x86_64-unknown-linux-gnu (nightly)	2m 3s
test x86_64-apple-darwin (stable)	3m 41s
test x86_64-apple-darwin (beta)	3m 3s
test x86_64-apple-darwin (nightly)	3m 0s
test x86_64-pc-windows-gnu (stable)	3m 42s
test i686-pc-windows-msvc (stable)	4m 47s
clippy	1m 6s
miri	2m 7s
fmt	27s
audit	0s
deny	1m 29s
udeps	7m 49s
upgrades	0s
book	3m 13s
gh-pages	0s
	1h 7m 46s

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