lints: Enable `warn(unused_qualifications)` #483

Job	Run time
cargo test msrv	1m 44s
Docs (macOS-latest)	20s
cargo test nightly (macOS-latest)	24s
cargo fmt	3s
Docs (windows-2019)	48s
cargo test nightly (windows-2019)	49s
cargo test nightly (ubuntu-latest)	24s
Docs (ubuntu-latest)	19s
cargo clippy+test (macOS-latest)	1m 0s
cargo clippy+test (wasm32) (macOS-latest)	28s
cargo clippy+test (windows-2019)	1m 50s
cargo clippy+test (wasm32) (windows-2019)	1m 47s
cargo clippy+test (ubuntu-latest)	50s
cargo clippy+test (wasm32) (ubuntu-latest)	28s
	11m 14s

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