Merge `ShardQueuer` functionality into `ShardManager` #1091

Job	Run time
Test (macOS)	1m 14s
Check minimal versions	26s
Test (nightly)	1m 10s
Test (stable)	2m 7s
Build docs	20s
Test (beta)	1m 39s
Examples	1m 53s
Test (Windows)	4m 26s
MSRV	0s
Test (native-tls)	1m 45s
Test (no default features)	35s
Test (no cache)	1m 46s
Test (no gateway)	1m 18s
Test (unstable Discord API features)	1m 56s
Test (zlib compression)	1m 51s
Test (zstd compression)	1m 46s
Test (zlib and zstd compression)	2m 10s
Test (chrono, chrono)	37s
Test (unstable API + typesize, default unstable typesize, true)	1m 2s
Test (builder without model, builder, true)	42s
Test (unstable Discord API (no default features), unstable, true)	24s
	29m 7s

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