Make `ImageHash` usable as key in `std` containers (#3321) #6096

Job	Run time
Test (macOS)	1m 39s
Build docs	52s
Check minimal versions	33s
MSRV	1m 3s
Test (nightly)	2m 53s
Test (stable)	2m 53s
Examples	4m 18s
Test (beta)	2m 51s
Test (Windows)	7m 31s
Test (native-tls)	2m 30s
Test (no default features)	49s
Test (no cache)	2m 31s
Test (no gateway)	1m 42s
Test (unstable Discord API features)	2m 29s
Test (simd-json)	2m 39s
Test (chrono, chrono)	49s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 11s
Test (builder without model, builder, true)	38s
Test (unstable Discord API (no default features), unstable_discord_api, true)	35s
	40m 26s

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