Change MessageUpdateEvent to just contain a full Message #1120

Job	Run time
Build docs	53s
Examples	2m 58s
Test (macOS)	1m 45s
Test (nightly)	1m 54s
MSRV	0s
Check minimal versions	57s
Test (stable)	2m 9s
Test (beta)	2m 26s
Test (Windows)	6m 20s
Test (native-tls)	2m 28s
Test (no default features)	37s
Test (no cache)	1m 39s
Test (no gateway)	1m 31s
Test (unstable Discord API features)	2m 26s
Test (zlib compression)	1m 56s
Test (zstd compression)	1m 51s
Test (zlib and zstd compression)	2m 10s
Test (chrono, chrono)	54s
Test (unstable API + typesize, default unstable typesize, true)	1m 9s
Test (builder without model, builder, true)	39s
Test (unstable Discord API (no default features), unstable, true)	44s
	37m 26s

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