Merge `ShardQueuer` functionality into `ShardManager` #1096

Job	Run time
Check minimal versions	53s
Build docs	50s
Test (macOS)	1m 48s
Examples	2m 54s
MSRV	0s
Test (nightly)	1m 49s
Test (stable)	2m 16s
Test (beta)	2m 32s
Test (Windows)	6m 2s
Test (native-tls)	2m 4s
Test (no default features)	55s
Test (no cache)	1m 57s
Test (no gateway)	1m 37s
Test (unstable Discord API features)	2m 31s
Test (zlib compression)	2m 7s
Test (zstd compression)	2m 9s
Test (zlib and zstd compression)	2m 9s
Test (chrono, chrono)	54s
Test (unstable API + typesize, default unstable typesize, true)	1m 4s
Test (builder without model, builder, true)	41s
Test (unstable Discord API (no default features), unstable, true)	38s
	37m 50s

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