Rename `Context::shard_manager` back to `shard` #1094

Job	Run time
Build docs	27s
Examples	2m 37s
Test (macOS)	1m 19s
Check minimal versions	31s
MSRV	0s
Test (nightly)	1m 29s
Test (stable)	2m 5s
Test (beta)	1m 56s
Test (Windows)	4m 19s
Test (native-tls)	1m 50s
Test (no default features)	46s
Test (no cache)	1m 42s
Test (no gateway)	1m 23s
Test (unstable Discord API features)	2m 0s
Test (zlib compression)	1m 52s
Test (zstd compression)	2m 0s
Test (zlib and zstd compression)	2m 7s
Test (chrono, chrono)	46s
Test (unstable API + typesize, default unstable typesize, true)	57s
Test (builder without model, builder, true)	17s
Test (unstable Discord API (no default features), unstable, true)	22s
	30m 45s

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