Use IndexMap instead of OrderedMap (#276) #1072

Job	Run time
Clippy	53s
Test (MSRV)	1m 15s
Build docs	2m 5s
Test (stable)	2m 10s
Rustfmt	4s
Test (beta)	2m 13s
Test (nightly)	1m 41s
Test (macOS)	2m 49s
Test (Windows)	4m 14s
Test (no features)	1m 13s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 25s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 28s
	21m 30s

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