Add `Level` type (#655) #1726

Job	Run time
check	1m 9s
fuzz record_ref	6m 49s
fuzz select_query	8m 28s
test x86_64-unknown-linux-gnu (stable)	2m 46s
test x86_64-unknown-linux-gnu (beta)	2m 15s
test x86_64-unknown-linux-gnu (nightly)	2m 40s
test x86_64-apple-darwin (stable)	3m 36s
test x86_64-apple-darwin (beta)	3m 59s
test x86_64-apple-darwin (nightly)	3m 4s
test x86_64-pc-windows-gnu (stable)	4m 32s
test i686-pc-windows-msvc (stable)	5m 35s
cross x86_64-unknown-linux-gnu (stable)	1m 45s
cross x86_64-unknown-linux-gnu (beta)	1m 54s
cross x86_64-unknown-linux-gnu (nightly)	2m 18s
cross x86_64-unknown-linux-musl (stable)	1m 44s
cross aarch64-unknown-linux-gnu (stable)	1m 31s
cross aarch64-unknown-linux-musl (stable)	1m 45s
clippy	1m 34s
audit	51s
deny	1m 46s
fmt	22s
book	3m 13s
miri	2m 8s
upgrades	2m 19s
udeps	7m 48s
gh-pages	5m 14s
	1h 21m 5s

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