Change `Embed::fields` to use `FixedArray` (#2674) #863

Job	Run time
Test (stable)	3m 3s
Test (beta)	3m 19s
Test (Windows)	6m 31s
Test (native-tls)	2m 52s
Test (no default features)	33s
Test (no cache)	2m 42s
Test (no gateway)	1m 43s
Test (unstable Discord API features)	3m 10s
Test (simd-json)	3m 10s
Test (chrono, chrono)	45s
Build docs	35s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	55s
Test (builder without model, builder, true)	23s
Test (unstable Discord API (no default features), unstable_discord_api, true)	25s
Examples	4m 5s
Check minimal versions	49s
Test (macOS)	6m 38s
Test (nightly)	2m 49s
MSRV	0s
	44m 27s

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