Add Context::guild_channel() #878

Job	Run time
Build docs	1m 52s
Clippy	1m 14s
Test (MSRV)	5m 56s
Rustfmt	16s
Test (stable)	3m 37s
Test (beta)	4m 29s
Test (nightly)	3m 33s
Test (macOS)	5m 41s
Test (Windows)	6m 17s
Test (no features)	2m 1s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	2m 7s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	3m 0s
	40m 3s

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