Replace declarative macros for generating argument parsers #1276

Job	Run time
Build docs	1m 29s
Clippy	54s
Rustfmt	5s
Test (MSRV)	1m 24s
Test (Windows)	3m 40s
Test (no features)	1m 30s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 22s
Test (beta)	2m 2s
Test (stable)	1m 40s
Test (macOS)	1m 50s
Test (nightly)	1m 58s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 23s
	19m 17s

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