Rewrite permission checks (#325) #1188

Job	Run time
Clippy	1m 46s
Build docs	1m 53s
Rustfmt	13s
Test (MSRV)	1m 24s
Test (stable)	2m 14s
Test (beta)	2m 8s
Test (nightly)	1m 52s
Test (macOS)	1m 30s
Test (Windows)	4m 46s
Test (no features)	1m 19s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 37s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 48s
	22m 30s

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