Split core functionality into a `serenity-core` subcrate #6386

Job	Run time
Examples	2m 33s
Check minimal versions	34s
Check (temp cache)	56s
Test (macOS)	1m 23s
Test (no cache)	49s
Test (nightly)	39s
Test (stable)	58s
Check (builder without model)	36s
Check (unstable (no default features))	45s
Test (no gateway)	1m 13s
Test (beta)	49s
Test (Windows)	3m 47s
Check (temp cache + typesize)	1m 5s
Test (no default features)	37s
Test (chrono)	41s
Build docs	42s
Test (zlib compression)	1m 9s
MSRV	0s
Check (unstable + typesize)	42s
Test (zlib and zstd compression)	1m 11s
Test (zstd compression)	1m 0s
Test (native-tls)	40s
	22m 49s

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