Adds "unstable-exhaustive-types" as new feature. #14

Job	Run time
Build docs	1m 29s
Clippy	46s
Rustfmt	4s
Test (MSRV)	1m 31s
Test (stable)	1m 57s
Test (beta)	2m 8s
Test (nightly)	1m 50s
Test (macOS)	1m 28s
Test (Windows)	4m 16s
Test (no features)	1m 8s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 21s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 14s
	19m 12s

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