Swap Command and related structs to avoid String allocs #1154

Job	Run time
Rustfmt	10s
Build docs	1m 21s
Test (MSRV)	1m 14s
Clippy	51s
Test (stable)	2m 1s
Test (beta)	2m 2s
Test (nightly)	1m 41s
Test (macOS)	1m 26s
Test (Windows)	4m 5s
Test (no features)	1m 11s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 15s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 30s
	18m 47s

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