Upgrade to darling 0.20 and syn v2 (#222) #908

Job	Run time
Rustfmt	15s
Clippy	54s
Test (MSRV)	5m 16s
Build docs	2m 31s
Test (stable)	1m 59s
Test (beta)	1m 59s
Test (nightly)	1m 59s
Test (macOS)	2m 27s
Test (Windows)	3m 48s
Test (no features)	1m 12s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 22s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 19s
	25m 1s

Provide feedback