Fix warnings to unblock Clippy CI (#306) #1130

Job	Run time
Rustfmt	4s
Clippy	44s
Build docs	2m 21s
Test (MSRV)	1m 26s
Test (stable)	2m 1s
Test (beta)	2m 4s
Test (nightly)	1m 39s
Test (macOS)	2m 35s
Test (Windows)	4m 3s
Test (no features)	1m 12s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 17s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 21s
	20m 47s

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