Swap Command and related structs to avoid String allocs #1190

Job	Run time
Rustfmt	12s
Clippy	1m 22s
Test (MSRV)	1m 28s
Build docs	1m 34s
Test (stable)	2m 2s
Test (beta)	2m 17s
Test (nightly)	1m 47s
Test (macOS)	1m 48s
Test (Windows)	5m 11s
Test (no features)	1m 20s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 33s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 33s
	22m 7s

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