Remove temporary `Vec` usage in the `http` module (#2624) #777

# to view logs

Job	Run time
Check minimal versions	34s
Test (macOS)	2m 44s
Test (nightly)	2m 58s
Test (stable)	3m 21s
Test (beta)	3m 26s
Build docs	34s
Test (Windows)	4m 58s
Examples	3m 37s
Test (native-tls)	2m 48s
Test (no default features)	53s
MSRV	0s
Test (no cache)	2m 50s
Test (no gateway)	1m 43s
Test (unstable Discord API features)	3m 6s
Test (simd-json)	3m 3s
Test (chrono, chrono)	50s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 6s
Test (builder without model, builder, true)	38s
Test (unstable Discord API (no default features), unstable_discord_api, true)	28s
	39m 37s

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