feat: store K in `Dict` entries to improve keys iterations efficiency #1639

Job	Run time
cargo-lint / cargo-dry-publish / publish	22s
cargo-lint / cargo-doc / doc	29s
cargo-lint / cargo-semver / Check semver	44s
cargo-lint / cargo-nursery-base / cargo-udeps / Check udeps	2m 28s
cargo-lint / cargo-nursery-base / cargo-audit / Audit	6m 36s
cargo-lint / cargo-nursery-base / cargo-lint / lint	2m 23s
cargo-publish	0s
cleanup	0s
	13m 2s

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