Perform some misc TODO cleanups #5780

Job	Run time
Test (macOS)	1m 10s
Test (stable)	2m 26s
Test (beta)	2m 7s
Build docs	26s
Test (Windows)	3m 50s
Examples	2m 4s
Test (native-tls)	2m 7s
Check minimal versions	27s
Test (no default features)	1m 8s
MSRV	0s
Test (no cache)	2m 19s
Test (nightly)	1m 13s
Test (no gateway)	1m 41s
Test (unstable Discord API features)	2m 19s
Test (chrono, chrono)	34s
Test (unstable API + typesize, default unstable typesize, true)	43s
Test (builder without model, builder, true)	46s
Test (unstable Discord API (no default features), unstable, true)	16s
	25m 36s

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