Split core functionality into a `serenity-core` subcrate #6377

Job	Run time
Check (unstable (no default features))	35s
Check (unstable + typesize)	53s
Check minimal versions	48s
Build docs	35s
Examples	1m 43s
Check (temp cache + typesize)	48s
Check (builder without model)	38s
Test (nightly)	56s
Test (no default features)	38s
Test (no gateway)	1m 1s
Test (macOS)	1m 31s
Check (temp cache)	46s
Test (native-tls)	52s
Test (Windows)	2m 54s
Test (beta)	1m 11s
Test (no cache)	58s
Test (zlib and zstd compression)	1m 12s
Test (stable)	1m 11s
Test (zstd compression)	1m 13s
Test (zlib compression)	1m 5s
Test (chrono)	33s
MSRV	0s
	22m 1s

Provide feedback