Fix warnings to unblock Clippy CI #1129

Job	Run time
Rustfmt	4s
Clippy	43s
Build docs	1m 26s
Test (MSRV)	1m 17s
Test (stable)	2m 1s
Test (beta)	2m 5s
Test (nightly)	1m 58s
Test (macOS)	2m 33s
Test (Windows)	5m 23s
Test (no features)	1m 7s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 19s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 14s
	21m 10s

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