Adds "unstable-exhaustive-types" as new feature. #4

Job	Run time
Test (macOS)	1m 17s
Check minimal versions	44s
Build docs	35s
Examples	2m 8s
MSRV	0s
Test (nightly)	1m 29s
Test (stable)	2m 26s
Test (beta)	2m 22s
Test (Windows)	4m 26s
Test (native-tls)	2m 14s
Test (no default features)	44s
Test (no cache)	2m 2s
Test (no gateway)	1m 37s
Test (unstable Discord API features)	2m 26s
Test (chrono, chrono)	34s
Test (unstable API + typesize, default unstable typesize, true)	56s
Test (builder without model, builder, true)	18s
Test (unstable Discord API (no default features), unstable, true)	17s
	26m 35s

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