Split core functionality into a `serenity-core` subcrate #6385

Job	Run time
Test (beta)	1m 11s
Check minimal versions	53s
Check (builder without model)	45s
Test (nightly)	1m 13s
Test (Windows)	3m 25s
Check (temp cache)	1m 0s
Check (temp cache + typesize)	56s
Test (no gateway)	54s
Build docs	1m 2s
Check (unstable (no default features))	37s
Test (no cache)	56s
Test (stable)	1m 25s
Test (native-tls)	1m 2s
Test (macOS)	1m 27s
Examples	2m 36s
Test (zstd compression)	1m 7s
Test (no default features)	43s
Test (chrono)	49s
Test (zlib and zstd compression)	1m 10s
Test (zlib compression)	1m 16s
Check (unstable + typesize)	49s
MSRV	-1s
	25m 15s

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