Clean up permissions.rs (#3046) #1010

Job	Run time
Examples	3m 42s
Build docs	1m 1s
Test (macOS)	1m 29s
MSRV	1m 3s
Check minimal versions	1m 7s
Test (nightly)	3m 35s
Test (stable)	3m 21s
Test (beta)	2m 52s
Test (Windows)	5m 10s
Test (native-tls)	2m 52s
Test (no default features)	55s
Test (no cache)	3m 9s
Test (no gateway)	1m 58s
Test (unstable Discord API features)	3m 0s
Test (simd-json)	3m 7s
Test (chrono, chrono)	39s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 2s
Test (builder without model, builder, true)	31s
Test (unstable Discord API (no default features), unstable_discord_api, true)	24s
	40m 57s

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