Fix #1328: optimize `JsonPointer.head()` #2372

Job	Run time
build (8, ubuntu-22.04)	1m 9s
build (11, ubuntu-22.04)	1m 5s
build (17, ubuntu-22.04)	57s
build (21, ubuntu-22.04)	53s
build (22, ubuntu-22.04)	1m 4s
build (8, windows-latest, W)	1m 47s
Trigger downstream builds for v3	0s
Trigger downstream builds for v2	0s
	6m 55s

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