Upgrade edition from 2018 to 2021 (#280) #18

Job	Run time
Clippy	48s
Build docs	1m 45s
Rustfmt	4s
Test (MSRV)	1m 18s
Test (stable)	2m 4s
Test (beta)	2m 4s
Test (nightly)	1m 38s
Test (macOS)	1m 29s
Test (Windows)	5m 15s
Test (no features)	1m 10s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 13s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 18s
	20m 6s

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