Merge `ShardQueuer` functionality into `ShardManager` #1107

Job	Run time
Test (macOS)	2m 2s
Test (nightly)	1m 53s
Test (stable)	2m 13s
Build docs	49s
Test (beta)	2m 24s
Examples	3m 4s
Check minimal versions	56s
Test (Windows)	4m 24s
MSRV	0s
Test (native-tls)	2m 11s
Test (no default features)	51s
Test (no cache)	1m 59s
Test (no gateway)	1m 32s
Test (unstable Discord API features)	2m 12s
Test (zlib compression)	1m 52s
Test (zstd compression)	2m 12s
Test (zlib and zstd compression)	2m 8s
Test (chrono, chrono)	55s
Test (unstable API + typesize, default unstable typesize, true)	1m 10s
Test (builder without model, builder, true)	43s
Test (unstable Discord API (no default features), unstable, true)	43s
	36m 13s

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