Swap to native async traits where possible #1215

Job	Run time
Build docs	1m 27s
Clippy	50s
Test (MSRV)	1m 16s
Rustfmt	8s
Test (stable)	1m 56s
Test (beta)	2m 3s
Test (nightly)	1m 52s
Test (macOS)	1m 45s
Test (Windows)	4m 49s
Test (no features)	1m 3s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 11s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 21s
	19m 41s

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