Factor out collector methods on models into traits #5827

Job	Run time
Test (macOS)	1m 42s
Build docs	33s
Test (stable)	2m 37s
Check minimal versions	28s
MSRV	0s
Test (beta)	2m 53s
Examples	2m 36s
Test (Windows)	5m 12s
Test (nightly)	1m 40s
Test (native-tls)	2m 30s
Test (no default features)	52s
Test (no cache)	2m 29s
Test (no gateway)	1m 50s
Test (unstable Discord API features)	2m 56s
Test (chrono, chrono)	1m 3s
Test (unstable API + typesize, default unstable typesize, true)	1m 6s
Test (builder without model, builder, true)	34s
Test (unstable Discord API (no default features), unstable, true)	36s
	31m 37s

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