Add a `BitPack` instance for `Char` #3573

Job	Run time
Build and run limited tests (macOS-13, 9.2)	18m 17s
Build and run limited tests (macOS-13, 9.4)	22m 1s
Build and run limited tests (macOS-13, 9.6)	23m 10s
Build and run limited tests (macOS-13, 9.10)	25m 46s
Build and run limited tests (windows-latest, 9.0)	16m 54s
Build and run limited tests (windows-latest, 8.10)	20m 57s
Build and run limited tests (macOS-13, 9.0)	19m 55s
Build and run limited tests (windows-latest, 9.2)	16m 47s
Build and run limited tests (windows-latest, 9.4)	18m 1s
Build and run limited tests (macOS-13, 9.8)	22m 46s
Build and Test	0s
All jobs finished	14s
	3h 24m 48s

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