Change MessageUpdateEvent to just contain a full Message #5829

Job	Run time
Test (macOS)	1m 39s
Test (nightly)	1m 32s
Build docs	37s
Examples	2m 44s
MSRV	0s
Check minimal versions	31s
Test (stable)	2m 31s
Test (beta)	2m 54s
Test (Windows)	4m 53s
Test (native-tls)	2m 21s
Test (no default features)	58s
Test (no cache)	2m 22s
Test (no gateway)	1m 39s
Test (unstable Discord API features)	2m 39s
Test (chrono, chrono)	52s
Test (unstable API + typesize, default unstable typesize, true)	1m 2s
Test (builder without model, builder, true)	48s
Test (unstable Discord API (no default features), unstable, true)	31s
	30m 33s

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