Implement inline choice (#232) #954

Job	Run time
Clippy	53s
Rustfmt	12s
Test (MSRV)	1m 19s
Build docs	2m 20s
Test (stable)	1m 57s
Test (beta)	2m 2s
Test (nightly)	2m 5s
Test (macOS)	4m 53s
Test (Windows)	5m 10s
Test (no features)	1m 23s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 16s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 13s
	24m 43s

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