feat: implement generics commands #900

Job	Run time
Clippy	49s
Rustfmt	5s
Test (MSRV)	1m 24s
Build docs	1m 23s
Test (stable)	2m 3s
Test (beta)	2m 0s
Test (nightly)	2m 5s
Test (macOS)	3m 2s
Test (Windows)	4m 43s
Test (no features)	1m 13s
Test (all features + simd_json, cargo test --all-features --features serenity/simd_json --lib --t...	1m 20s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 19s
	21m 26s

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