Fix model functions using `SerializeIter` (#2955) #5522

Job	Run time
Build docs	41s
Examples	2m 36s
Test (macOS)	1m 25s
Check minimal versions	30s
MSRV	0s
Test (stable)	2m 38s
Test (nightly)	1m 42s
Test (beta)	2m 35s
Test (Windows)	4m 17s
Test (native-tls)	2m 18s
Test (no default features)	59s
Test (no cache)	2m 28s
Test (no gateway)	1m 41s
Test (unstable Discord API features)	2m 44s
Test (chrono, chrono)	1m 13s
Test (unstable API + typesize, default unstable typesize, true)	54s
Test (builder without model, builder, true)	34s
Test (unstable Discord API (no default features), unstable, true)	27s
	29m 42s

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