Factor out collector methods on models into traits #5835

Job	Run time
Build docs	38s
Examples	2m 55s
Test (macOS)	2m 7s
Check minimal versions	32s
MSRV	0s
Test (nightly)	1m 35s
Test (stable)	2m 35s
Test (beta)	2m 29s
Test (Windows)	4m 50s
Test (native-tls)	2m 36s
Test (no default features)	57s
Test (no cache)	2m 29s
Test (no gateway)	1m 53s
Test (unstable Discord API features)	2m 45s
Test (chrono, chrono)	1m 4s
Test (unstable API + typesize, default unstable typesize, true)	59s
Test (builder without model, builder, true)	37s
Test (unstable Discord API (no default features), unstable, true)	34s
	31m 35s

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