Encapsulate Message Cache in it's own type #1119

Job	Run time
Build docs	55s
Examples	3m 7s
Test (macOS)	1m 27s
Check minimal versions	58s
Test (nightly)	1m 30s
MSRV	0s
Test (stable)	1m 16s
Test (beta)	1m 22s
Test (Windows)	4m 5s
Test (native-tls)	1m 26s
Test (no default features)	1m 0s
Test (no cache)	2m 21s
Test (no gateway)	1m 55s
Test (unstable Discord API features)	1m 14s
Test (chrono, chrono)	55s
Test (unstable API + typesize, default unstable typesize, true)	59s
Test (builder without model, builder, true)	40s
Test (unstable Discord API (no default features), unstable, true)	46s
	25m 56s

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