Adds "unstable-exhaustive-types" as new feature. #1220

Job	Run time
Build docs	1m 26s
Rustfmt	4s
Test (MSRV)	1m 25s
Clippy	47s
Test (stable)	2m 1s
Test (beta)	2m 11s
Test (nightly)	1m 46s
Test (macOS)	1m 31s
Test (Windows)	4m 44s
Test (no features)	1m 9s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 14s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 16s
	19m 34s

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