Improve tokens semantics and clean up gateway feature gates #5793

Job	Run time
Test (nightly)	1m 23s
Test (stable)	2m 27s
Test (beta)	2m 13s
Examples	23s
Test (Windows)	4m 4s
Test (macOS)	1m 14s
Test (native-tls)	2m 28s
Test (no default features)	1m 3s
Build docs	25s
Check minimal versions	34s
Test (no cache)	2m 4s
Test (no gateway)	1m 37s
Test (unstable Discord API features)	2m 31s
Test (chrono, chrono)	52s
MSRV	0s
Test (unstable API + typesize, default unstable typesize, true)	42s
Test (builder without model, builder, true)	20s
Test (unstable Discord API (no default features), unstable, true)	16s
	24m 36s

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