Split core functionality into a `serenity-core` subcrate #6378

Job	Run time
Check (builder without model)	33s
Check minimal versions	44s
Check (temp cache)	41s
Check (temp cache + typesize)	1m 17s
Check (unstable + typesize)	58s
Build docs	41s
Check (unstable (no default features))	29s
Examples	1m 53s
Test (stable)	49s
Test (beta)	39s
Test (nightly)	45s
Test (no default features)	41s
Test (no cache)	58s
Test (Windows)	2m 4s
Test (native-tls)	42s
MSRV	-1s
Test (no gateway)	1m 10s
Test (zlib and zstd compression)	1m 1s
Test (zlib compression)	58s
Test (chrono)	39s
Test (macOS)	1m 2s
Test (zstd compression)	49s
	19m 32s

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