feat: implement generics commands (#219) #901

Job	Run time
Clippy	47s
Rustfmt	7s
Build docs	2m 36s
Test (MSRV)	1m 25s
Test (stable)	2m 0s
Test (beta)	1m 56s
Test (nightly)	2m 7s
Test (macOS)	5m 59s
Test (Windows)	3m 56s
Test (no features)	1m 15s
Test (all features + simd_json, cargo test --all-features --features serenity/simd_json --lib --t...	1m 18s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 24s
	24m 50s

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