Remove json_safe_u64 wrapper for Voice Heartbeat #915

Job	Run time
Build docs	29s
Examples	3m 27s
Test (macOS)	1m 32s
Check minimal versions	34s
Test (nightly)	3m 8s
MSRV	43s
Test (stable)	3m 7s
Test (beta)	3m 43s
Test (Windows)	4m 40s
Test (native-tls)	2m 49s
Test (no default features)	46s
Test (no cache)	2m 49s
Test (no gateway)	2m 1s
Test (unstable Discord API features)	3m 5s
Test (simd-json)	3m 10s
Test (chrono, chrono)	35s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	55s
Test (builder without model, builder, true)	28s
Test (unstable Discord API (no default features), unstable_discord_api, true)	17s
	38m 18s

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