Add `KEEP` support #141

Job	Run time
msrv	2m 52s
Check format	6s
Check clippy	19s
Test and release for x86_64-pc-windows-gnu	3m 19s
Test and release for x86_64-pc-windows-msvc	3m 23s
Test and release for x86_64-apple-darwin	1m 28s
Test and release for aarch64-apple-darwin	54s
Test and release for x86_64-unknown-linux-musl	1m 0s
Test and release for x86_64-unknown-linux-gnu	57s
Publish	36s
	14m 54s

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